[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

llvm-svn: 308208
diff --git a/llvm/utils/vim/syntax/llvm.vim b/llvm/utils/vim/syntax/llvm.vim
index a073d62..d047578 100644
--- a/llvm/utils/vim/syntax/llvm.vim
+++ b/llvm/utils/vim/syntax/llvm.vim
@@ -161,7 +161,7 @@
       \ within
       \ writeonly
       \ x86_64_sysvcc
-      \ x86_64_win64cc
+      \ win64cc
       \ x86_fastcallcc
       \ x86_stdcallcc
       \ x86_thiscallcc