[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/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 83682a4..a651bdb 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -2659,6 +2659,7 @@
   case CC_X86RegCall:
   case CC_AAPCS:
   case CC_AAPCS_VFP:
+  case CC_AArch64VectorCall:
   case CC_IntelOclBicc:
   case CC_SpirFunction:
   case CC_OpenCLKernel: