Add frontend support for __vectorcall

Wire it through everywhere we have support for fastcall, essentially.

This allows us to parse the MSVC "14" CTP headers, but we will
miscompile them because LLVM doesn't support __vectorcall yet.

Reviewed By: Aaron Ballman

Differential Revision: http://reviews.llvm.org/D5808

llvm-svn: 220573
diff --git a/clang/test/CodeGenCXX/mangle-ms.cpp b/clang/test/CodeGenCXX/mangle-ms.cpp
index 3285c98..bf61d8b 100644
--- a/clang/test/CodeGenCXX/mangle-ms.cpp
+++ b/clang/test/CodeGenCXX/mangle-ms.cpp
@@ -365,3 +365,5 @@
 // CHECK-DAG: ??3TypedefNewDelete@@SAXPAX@Z
 // CHECK-DAG: ??_VTypedefNewDelete@@SAXPAX@Z
 
+void __vectorcall vector_func() { }
+// CHECK-DAG: @"\01?vector_func@@YQXXZ"