Fix for #31362 - ms_abi is implemented incorrectly for values >=16 bytes.

Summary:
This patch is a fix for following issue:
https://bugs.llvm.org/show_bug.cgi?id=31362 The problem was caused by front end
lowering C calling conventions without taking into account calling conventions
enforced by attribute. In this case win64cc was no correctly lowered on targets
other than Windows.

Reviewed By: rnk (Reid Kleckner)

Differential Revision: https://reviews.llvm.org/D43016

Author: belickim <mateusz.belicki@intel.com>
llvm-svn: 324594