[MS] Add metadata for __declspec(allocator)

Summary:
Emit !heapallocsite in the metadata for calls to functions marked with
__declspec(allocator). Eventually this will be emitted as S_HEAPALLOCSITE debug
info in codeview.

Reviewers: rnk

Subscribers: jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60237

llvm-svn: 357928