[ms-cxxabi] Fix the calling convention for operator new in records

Summary:
Operator new, new[], delete, and delete[] are all implicitly static when
declared inside a record.  CXXMethodDecl already knows this, but we need
to account for that before we pick the calling convention for the
function type.

Fixes PR17371.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1761