[AArch64] Add aarch64_vector_pcs function attribute to Clang

This is the Clang patch to complement the following LLVM patches:
  https://reviews.llvm.org/D51477
  https://reviews.llvm.org/D51479

More information describing the vector ABI and procedure call standard
can be found here:

https://developer.arm.com/products/software-development-tools/\
                          hpc/arm-compiler-for-hpc/vector-function-abi

Patch by Kerry McLaughlin.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D54425

llvm-svn: 347571
diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index aaacb8d..c51dfb1 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -3336,6 +3336,7 @@
   CXCallingConv_Swift = 13,
   CXCallingConv_PreserveMost = 14,
   CXCallingConv_PreserveAll = 15,
+  CXCallingConv_AArch64VectorCall = 16,
 
   CXCallingConv_Invalid = 100,
   CXCallingConv_Unexposed = 200