sanitizer_common: add FALLTHROUGH macro

[dvyukov]

Summary

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.

[dvyukov]

changed SelectMapping to use template Arg and rebased

compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h

 # define __has_feature(x) 0
 #endif
 
 // Older GCCs do not understand __has_attribute.
 #if !defined(__has_attribute)
 # 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.
 namespace __sanitizer {
 
 #if defined(_WIN64)
 // 64-bit Windows uses LLP64 data model.
 typedef unsigned long long uptr;

 #endif
 
 #if !defined(_MSC_VER) || defined(__clang__) || MSC_PREREQ(1900)
 # define NOEXCEPT noexcept
 #else
 # 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;
 typedef ALIGNED(1) u64 uu64;
 typedef ALIGNED(1) s16 us16;
 typedef ALIGNED(1) s32 us32;
 typedef ALIGNED(1) s64 us64;