[AArch64] Extend CallingConv::X86_64_Win64 to AArch64 as well

Rename the enum value from X86_64_Win64 to plain Win64.

The symbol exposed in the textual IR is changed from 'x86_64_win64cc'
to 'win64cc', but the numeric value is kept, keeping support for
old bitcode.

Differential Revision: https://reviews.llvm.org/D34474

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308208 91177308-0d34-0410-b5e6-96231b3b80d8
31 files changed