Correct handling of _Pragma macro inside a macro argument.

If we are pre-expanding a macro argument don't actually "activate"
the pragma at that point, activate the pragma whenever we encounter
it again in the token stream.
This ensures that we will activate it in the correct location
or that we will ignore it if it never enters the token stream, e.g:

     \#define EMPTY(x)
     \#define INACTIVE(x) EMPTY(x)
     INACTIVE(_Pragma("clang diagnostic ignored \"-Wconversion\""))

This also fixes the crash in rdar://11168596.

llvm-svn: 153959