regcall: Implement regcall Calling Conv in clang

This patch implements the register call calling convention, which ensures
as many values as possible are passed in registers. CodeGen changes
were committed in https://reviews.llvm.org/rL284108.

Differential Revision: https://reviews.llvm.org/D25204

llvm-svn: 285849
diff --git a/clang/lib/AST/Expr.cpp b/clang/lib/AST/Expr.cpp
index a23904d..9cf9e86 100644
--- a/clang/lib/AST/Expr.cpp
+++ b/clang/lib/AST/Expr.cpp
@@ -543,6 +543,7 @@
       case CC_X86FastCall: POut << "__fastcall "; break;
       case CC_X86ThisCall: POut << "__thiscall "; break;
       case CC_X86VectorCall: POut << "__vectorcall "; break;
+      case CC_X86RegCall: POut << "__regcall "; break;
       // Only bother printing the conventions that MSVC knows about.
       default: break;
       }