re2: use C++17 [[fallthrough]] instead of ABSL_FALLTHROUGH_INTENDED

According to [1], Google code now depends on C++17,
so we can use these directly.

[1] https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

Change-Id: Ic475b79bc2153861d6ec2aebc0c55dbcf0c35908
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63770
Reviewed-by: Alan Donovan <adonovan@google.com>

re2/bitmap256.cc

diff --git a/re2/bitmap256.cc b/re2/bitmap256.cc
index 4b9eee4..d3f8087 100644
--- a/re2/bitmap256.cc
+++ b/re2/bitmap256.cc
@@ -6,7 +6,6 @@
 
 #include <stdint.h>
 
-#include "absl/base/attributes.h"
 #include "absl/log/absl_check.h"
 
 namespace re2 {
@@ -27,15 +26,15 @@
     case 1:
       if (words_[1] != 0)
         return (1 * 64) + FindLSBSet(words_[1]);
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case 2:
       if (words_[2] != 0)
         return (2 * 64) + FindLSBSet(words_[2]);
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case 3:
       if (words_[3] != 0)
         return (3 * 64) + FindLSBSet(words_[3]);
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     default:
       return -1;
   }

re2/dfa.cc

diff --git a/re2/dfa.cc b/re2/dfa.cc
index 1619614..712a7b6 100644
--- a/re2/dfa.cc
+++ b/re2/dfa.cc
@@ -34,7 +34,6 @@
 #include <utility>
 #include <vector>
 
-#include "absl/base/attributes.h"
 #include "absl/base/call_once.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/container/flat_hash_map.h"
@@ -650,7 +649,7 @@
             absl::FPrintF(stderr, " -> FullMatchState\n");
           return FullMatchState;
         }
-        ABSL_FALLTHROUGH_INTENDED;
+        [[fallthrough]];
       default:
         // Record iff id is the head of its list, which must
         // be the case if id-1 is the last of *its* list. :)

re2/parse.cc

diff --git a/re2/parse.cc b/re2/parse.cc
index 6f7a561..545b965 100644
--- a/re2/parse.cc
+++ b/re2/parse.cc
@@ -24,7 +24,6 @@
 #include <string>
 #include <vector>
 
-#include "absl/base/attributes.h"
 #include "absl/base/macros.h"
 #include "absl/log/absl_log.h"
 #include "absl/strings/ascii.h"
@@ -304,7 +303,7 @@
     case EvenOddSkip:  // even <-> odd but only applies to every other
       if ((r - f->lo) % 2)
         return r;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case EvenOdd:  // even <-> odd
       if (r%2 == 0)
         return r + 1;
@@ -313,7 +312,7 @@
     case OddEvenSkip:  // odd <-> even but only applies to every other
       if ((r - f->lo) % 2)
         return r;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case OddEven:  // odd <-> even
       if (r%2 == 1)
         return r + 1;
@@ -1525,7 +1524,7 @@
       // Single non-zero octal digit is a backreference; not supported.
       if (s->empty() || (*s)[0] < '0' || (*s)[0] > '7')
         goto BadEscape;
-      ABSL_FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case '0':
       // consume up to three octal digits; already have one.
       code = c - '0';

re2/prog.cc

diff --git a/re2/prog.cc b/re2/prog.cc
index 9f1cc00..c60d930 100644
--- a/re2/prog.cc
+++ b/re2/prog.cc
@@ -15,7 +15,6 @@
 #include <utility>
 #include <vector>
 
-#include "absl/base/attributes.h"
 #include "absl/log/absl_check.h"
 #include "absl/log/absl_log.h"
 #include "absl/strings/str_format.h"
@@ -820,7 +819,7 @@
         flat->back().set_opcode(kInstAltMatch);
         flat->back().set_out(static_cast<int>(flat->size()));
         flat->back().out1_ = static_cast<uint32_t>(flat->size())+1;
-        ABSL_FALLTHROUGH_INTENDED;
+        [[fallthrough]];
 
       case kInstAlt:
         stk->push_back(ip->out1());

re2/walker-inl.h

diff --git a/re2/walker-inl.h b/re2/walker-inl.h
index 2283d6d..07c9aef 100644
--- a/re2/walker-inl.h
+++ b/re2/walker-inl.h
@@ -192,7 +192,7 @@
           s->child_args = &s->child_arg;
         else if (re->nsub_ > 1)
           s->child_args = new T[re->nsub_];
-        ABSL_FALLTHROUGH_INTENDED;
+        [[fallthrough]];
       }
       default: {
         if (re->nsub_ > 0) {