[clang][NFC] Adjust tests to not un/define predefined macros

An upcoming patch will be making all defining or undefining of
predefined macros to be warning (currently only some give a warning).
In preparation for this adjust some tests that would emit a warning:
 * In thread-specifier.c the undefine is done to avoid a different
   warning, but we get that warning just because __thread and
   __private_extern__ are the wrong way around so we can just swap
   them.
 * There are a couple of objective-c tests that redefine IBAction to
   what it's already defined as, so we can just remove the define.