PR #2136: Give externally-provided symbol declarations default visibility

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/2136

Abseil forward-declares a few symbols that are provided by something *outside* the object being linked:

| Symbol(s) | File | Provided by |
|---|---|---|
| 14x `Annotate*` race annotations | `absl/base/dynamic_annotations.h` | sanitizer runtime |
| `__tsan_read1` | `absl/synchronization/mutex.cc` | sanitizer runtime |
| `__mmap2` | `absl/base/internal/direct_mmap.h` | Bionic |

None of these has a public header to include — none of the `Annotate*` functions nor `__tsan_read1` appears in any compiler-rt `sanitizer/*.h`, and `__mmap2` is a private Bionic symbol — so declaring them locally is the only option. But a local declaration inherits whatever visibility is in effect at that point.

When the translation unit is compiled with `-fvisibility=hidden` and abseil is linked into a shared library, those references become *hidden* undefined symbols. They then cannot be resolved from the sanitizer runtime (which lives in the executable) or from libc, and the link fails:

```
ld.lld: error: undefined hidden symbol: AnnotateIgnoreReadsBegin
ld.lld: error: undefined hidden symbol: AnnotateIgnoreWritesBegin
ld.lld: error: undefined hidden symbol: AnnotateIgnoreWritesEnd
ld.lld: error: undefined hidden symbol: AnnotateIgnoreReadsEnd
ld.lld: error: undefined hidden symbol: __tsan_read1
ld.lld: error: undefined hidden symbol: __mmap2
```

This wraps each declaration in `#pragma GCC visibility push(default)` / `pop`.

### Verification

Compiling all ~163 abseil sources with `-fsanitize=thread` and a tree-wide `#pragma GCC visibility push(hidden)`, then scanning every object for hidden undefined symbols: before this change the four `Annotate*` and `__tsan_read1` are `GLOBAL HIDDEN UND`; after, every `__tsan_*` and `Annotate*` reference is `GLOBAL DEFAULT UND`. The only hidden undefined symbols remaining are `AbslInternal*` ones that abseil defines itself, which is correct.

For `__mmap2`, on a 32-bit Android shared-library link:

```
without the pragma:  ld.lld: error: undefined hidden symbol: __mmap2
with the pragma:     4: FUNC GLOBAL DEFAULT UND __mmap2@LIBC
```

### Notes

- Every affected site is already inside a Bionic or sanitizer guard, so the GCC/Clang-only pragma does not reduce portability.
- The `__mmap2` path only compiles on 32-bit architectures, which is why this is rarely hit — Chromium's Android builds are 64-bit. We hit it on 32-bit ARM.
- Found while updating the copy of abseil vendored into Firefox, which builds with `-fvisibility=hidden` tree-wide.
Merge ab78d3a7f471fdaa44f6b7bc5feece81cd1d2833 into e1cacc79e6a318108eded02261219ceabff38ce0

Merging this change closes #2136

PiperOrigin-RevId: 975244998
Change-Id: I1be645e8658b7a323fdcc34f081d3466cd9eb6ed

absl/base/dynamic_annotations.h

diff --git a/absl/base/dynamic_annotations.h b/absl/base/dynamic_annotations.h
index f18b5e0..1e679a6 100644
--- a/absl/base/dynamic_annotations.h
+++ b/absl/base/dynamic_annotations.h
@@ -202,6 +202,7 @@
 
 // Function prototypes of annotations provided by the compiler-based sanitizer
 // implementation.
+#pragma GCC visibility push(default)
 ABSL_INTERNAL_BEGIN_EXTERN_C
 void AnnotateRWLockCreate(const char* file, int line,
                           const volatile void* lock);
@@ -221,6 +222,7 @@
 void AnnotateThreadName(const char* file, int line, const char* name);
 void AnnotateEnableRaceDetection(const char* file, int line, int enable);
 ABSL_INTERNAL_END_EXTERN_C
+#pragma GCC visibility pop
 
 #else  // ABSL_INTERNAL_RACE_ANNOTATIONS_ENABLED == 0
 
@@ -297,12 +299,14 @@
 
 // Function prototypes of annotations provided by the compiler-based sanitizer
 // implementation.
+#pragma GCC visibility push(default)
 ABSL_INTERNAL_BEGIN_EXTERN_C
 void AnnotateIgnoreReadsBegin(const char* file, int line)
     ABSL_INTERNAL_IGNORE_READS_BEGIN_ATTRIBUTE;
 void AnnotateIgnoreReadsEnd(const char* file,
                             int line) ABSL_INTERNAL_IGNORE_READS_END_ATTRIBUTE;
 ABSL_INTERNAL_END_EXTERN_C
+#pragma GCC visibility pop
 
 #elif defined(ABSL_INTERNAL_ANNOTALYSIS_ENABLED)
 
@@ -353,10 +357,12 @@
 
 // Function prototypes of annotations provided by the compiler-based sanitizer
 // implementation.
+#pragma GCC visibility push(default)
 ABSL_INTERNAL_BEGIN_EXTERN_C
 void AnnotateIgnoreWritesBegin(const char* file, int line);
 void AnnotateIgnoreWritesEnd(const char* file, int line);
 ABSL_INTERNAL_END_EXTERN_C
+#pragma GCC visibility pop
 
 #else
 

absl/base/internal/direct_mmap.h

diff --git a/absl/base/internal/direct_mmap.h b/absl/base/internal/direct_mmap.h
index f56ba23..855f51e 100644
--- a/absl/base/internal/direct_mmap.h
+++ b/absl/base/internal/direct_mmap.h
@@ -54,7 +54,9 @@
 
 // SYS_mmap and SYS_munmap are not defined in Android.
 #ifdef __BIONIC__
+#pragma GCC visibility push(default)
 extern "C" void* __mmap2(void*, size_t, int, int, int, size_t);
+#pragma GCC visibility pop
 #if defined(__NR_mmap) && !defined(SYS_mmap)
 #define SYS_mmap __NR_mmap
 #endif

absl/synchronization/mutex.cc

diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index be0d91a..d6e2aac 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -2788,7 +2788,9 @@
 }
 
 #ifdef ABSL_HAVE_THREAD_SANITIZER
+#pragma GCC visibility push(default)
 extern "C" void __tsan_read1(void* addr);
+#pragma GCC visibility pop
 #else
 #define __tsan_read1(addr)  // do nothing if TSan not enabled
 #endif