sanitizer_common: add FALLTHROUGH macro

Add FALLTHROUGH portably defined to [[clang::fallthrough]].
We have -Wimplicit-fallthrough already enabled, and currently
it's not possible to fix the warning.

Depends on D107735.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107736

Author: dvyukov

compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h

@@ -125,6 +125,10 @@
 # define __has_attribute(x) 0
 #endif
 
+#if !defined(__has_cpp_attribute)
+#  define __has_cpp_attribute(x) 0
+#endif
+
 // For portability reasons we do not include stddef.h, stdint.h or any other
 // system header, but we do need some basic types that are not defined
 // in a portable way by the language itself.
@@ -250,6 +254,12 @@ typedef u64 tid_t;
 # define NOEXCEPT throw()
 #endif
 
+#if __has_cpp_attribute(clang::fallthrough)
+#  define FALLTHROUGH [[clang::fallthrough]]
+#else
+#  define FALLTHROUGH
+#endif
+
 // Unaligned versions of basic types.
 typedef ALIGNED(1) u16 uu16;
 typedef ALIGNED(1) u32 uu32;