vec: Silence -Wunused-function warnings on clang

vec: Silence -Wunused-function warnings on clang

clang has a too aggressive (or broken, depends on how you want to see
it) -Wunused-function warning, which is triggered by the functions
defined by DEF_VEC_* but not used in the current source file.  Normally,
it won't warn about unused static inline functions defined in header
files, because it's expected that a source file won't use all functions
defined in a header file it includes.  However, if the DEF_VEC_* macro
is used in a source file, it considers those functions as defined in the
source file, which leads it to think that we should remove those
functions.  It is therefore missing a check to see whether those
functions are resulting from macro expansion.  A bug already exists for
that:

  https://bugs.llvm.org//show_bug.cgi?id=22712

It's quite easy to silence this warning in a localized way, that is in
the DEF_VEC_* macros.

gdb/ChangeLog:

	* common/diagnostics.h: Define macros for GCC.
	(DIAGNOSTIC_IGNORE_UNUSED_FUNCTION): New macro.
	* common/vec.h: Include diagnostics.h.
	(DIAGNOSTIC_IGNORE_UNUSED_VEC_FUNCTION): New macro.
	(DEF_VEC_I, DEF_VEC_P, DEF_VEC_O): Ignore -Wunused-function
	warning.