Windows ARM: ignore calling conventions as described on MSDN

Summary:
MSDN says that fastcall, stdcall, thiscall, and vectorcall are all
accepted but ignored on ARM and X64.
https://msdn.microsoft.com/en-us/library/984x0h58.aspx

MSDN also says cdecl is also accepted and typically ignored
This patch brings ARM in line with how we ignore them for X64

Reviewers: rnk

Subscribers: compnerd, cfe-commits

Differential Revision: http://reviews.llvm.org/D12034

llvm-svn: 245076