Revert r308441 "Recommit r308327: Add a warning for missing '#pragma pack (pop)'...

Revert r308441 "Recommit r308327: Add a warning for missing '#pragma pack (pop)' and suspicious uses of '#pragma pack' in included files"

This seems to have broken the sanitizer-x86_64-linux buildbot. Reverting until
it's fixed, especially since this landed just before the 5.0 branch.

> This commit adds a new -Wpragma-pack warning. It warns in the following cases:
>
> - When a translation unit is missing terminating #pragma pack (pop) directives.
> - When entering an included file if the current alignment value as determined
>   by '#pragma pack' directives is different from the default alignment value.
> - When leaving an included file that changed the state of the current alignment
>   value.
>
> rdar://10184173
>
> Differential Revision: https://reviews.llvm.org/D35484

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308455 91177308-0d34-0410-b5e6-96231b3b80d8