Add LLVM_FALLTHROUGH macro to Compiler.h

[jasonmolenda]

Summary

I'd like to revive Peter's Compiler.h patch in r237766 from May of last year, defining an LLVM_FALLTHROUGH macro for use in the lldb codebase. It looks like we've got at least half a dozen instances where case statements are not terminated with 'break;' when they should be. I will mark up the places in lldb where we fall through intentionally with this macro and then add -Wimplicit-fallthrough to our clang compile flags; I'd rather add it to the llvm Compiler.h file so we're not duplicating this over in lldb.

[pcc]

LGTM

[Eugene.Zelenko]

Obsoleted by r278868.

include/llvm/Support/Compiler.h

 # define __has_attribute(x) 0
 #endif
 
+#ifndef __has_cpp_attribute
+# define __has_cpp_attribute(x) 0
+#endif
+
 #ifndef __has_builtin
 # define __has_builtin(x) 0
 #endif

 #define LLVM_THREAD_LOCAL
 #endif
 
+/// \macro LLVM_FALLTHROUGH
+/// \brief Marks an empty statement preceding a deliberate switch fallthrough.
+#if __has_cpp_attribute(clang::fallthrough)
+#define LLVM_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define LLVM_FALLTHROUGH
+#endif
+
 #endif