Ignore calling conventions when checking function template specializations

Summary:
Calling conventions are inherited during decl merging.  Before this
change, deduction would fail due to a type mismatch between the template
and the specialization.  This change adjusts the CCs to match before
deduction, and lets the decl merging logic diagnose mismatch or inherit
the CC from the template.

This allows specializations of static member function templates in the
Microsoft C++ ABI.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1570