tools: Fixes bug where clang-tidy assumes cc flags

A minor bug was introduced where the clang-tidy tooling would always
assume that it was compiling for c++ even when compiling a c source
file. This change fixes this bug, and additionally adds regression
tests.

.bazelrc

diff --git a/.bazelrc b/.bazelrc
index b6e4a9c..22c27d5 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -14,4 +14,7 @@
 coverage --combined_report=lcov
 coverage --experimental_use_llvm_covmap
 coverage --experimental_generate_llvm_lcov
-coverage --incompatible_cc_coverage
\ No newline at end of file
+coverage --incompatible_cc_coverage
+
+# Static analyser support 
+build:analyser --aspects //tools/clang_tidy:clang_tidy.bzl%clang_tidy_aspect --output_groups=report 
\ No newline at end of file

tests/BUILD.bazel

diff --git a/tests/BUILD.bazel b/tests/BUILD.bazel
index 05448f9..4888efd 100644
--- a/tests/BUILD.bazel
+++ b/tests/BUILD.bazel
@@ -21,3 +21,8 @@
     name = "dereferencing_null_pointer",
     srcs = ["dereferencing_null_pointer.cc"],
 )
+
+cc_binary(
+    name = "c_only",
+    srcs = ["c_only.c"],
+)

tests/c_only.c

diff --git a/tests/c_only.c b/tests/c_only.c
new file mode 100644
index 0000000..3ce2722
--- /dev/null
+++ b/tests/c_only.c
@@ -0,0 +1,6 @@
+#ifdef __cplusplus
+#error \
+    "This is the C file, this file should not be compiled with -xc++ \
+    language directives."
+#endif
+int main() { return 0; }
\ No newline at end of file

tools/clang_tidy/clang_tidy.bzl

diff --git a/tools/clang_tidy/clang_tidy.bzl b/tools/clang_tidy/clang_tidy.bzl
index 20b5ce7..b037ab1 100644
--- a/tools/clang_tidy/clang_tidy.bzl
+++ b/tools/clang_tidy/clang_tidy.bzl
@@ -16,7 +16,7 @@
     Args:
         file (File): The file to get the language for
     """
-    if file.extension == ".c":
+    if file.extension == "c":
         return "c"
     else:
         return "cc"