Expand macros in pragmas with -fms-extensions and -E

gcc never expands macros in pragmas and MSVC always expands macros
before processing pragmas.  Clang usually allows macro expansion, except
in a handful of pragmas, most of which are handled by the lexer.

Also remove PPCallbacks for pragmas that are currently handled in the
parser.  Without a Parser, such as with clang -E, these callbacks would
never be called.

Fixes PR18576.

llvm-svn: 201821