Add Sema implementation of #pragma pack stack.

Add Sema implementation of #pragma pack stack.
 - Follows the MSVC (original) implementation, including support of
   pack(show) (useful for testing).
 - Implements support for named pack records which gcc seems to
   ignore (or implements incorrectly).
 - Not currently wired to anything, only functionality change is the
   type checking of the pragma.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@57476 91177308-0d34-0410-b5e6-96231b3b80d8