Handle "#pragma GCC visibility" in a few more places. Switch over "#pragma...

Handle "#pragma GCC visibility" in a few more places.  Switch over "#pragma pack" to use the same handling that gcc does.  Fixes <rdar://problem/10871094> and <rdar://problem/10893316>.

(Hopefully, common usage of these pragmas isn't irregular enough to break our current handling.  Doug has ideas for a more crazy approach if necessary.)

llvm-svn: 151307