Recommit "[MS] Improved implementation of stack pragmas (vtordisp, *_seg)"

Slightly updated version, double-checked build and tests.
Improve implementation of MS pragmas that use stack + compatibility fixes.
This patch:
  1. Changes implementation of #pragma vtordisp to use PragmaStack class
     that other stack pragmas use;
  2. Fixes "#pragma vtordisp()" behavior - it shouldn't affect the stack;
  3. Supports "save-restore" of pragma stacks on enter / exit a C++ method
     body, as MSVC does.

TODO:
  1. Change implementation of #pragma pack to use the same approach;
  2. Introduce diagnostics on popping named stack slots, as MSVC does.

Reviewers:
  rnk, thakis

Differential revision: http://reviews.llvm.org/D19361

llvm-svn: 268029