[MS] Make #pragma pack use PragmaStack<> class.

Make implementation of #pragma pack consistent with other "stack" pragmas.
Use PragmaStack<> class instead of old representation of internal stack.
Don't change compiler's behavior.

TODO:
  1. Introduce diagnostics on popping named slots from pragma stacks.

Reviewer: rnk

Differential revision: http://reviews.llvm.org/D19727

llvm-svn: 268085