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

Rework implementation of several MS pragmas that use internal stack:
vtordisp, {bss|code|const|data}_seg.
This patch:
  1. Makes #pragma vtordisp use PragmaStack class as *_seg pragmas do;
  2. Fixes "#pragma vtordisp()" behavior: it shouldn't affect stack;
  3. Saves/restores the stacks on enter/exit a C++ method body.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267866 91177308-0d34-0410-b5e6-96231b3b80d8