Fix our handling of #pragma GCC visibility.

Fix our handling of #pragma GCC visibility.
Now the lexer just produces a token and the parser is the one responsible for
activating it.
This fixes problem like the one pr11797 where the lexer and the parser were not
in sync. This also let us be more strict on where in the file we accept
these pragmas.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149014 91177308-0d34-0410-b5e6-96231b3b80d8