Recommit "Add _Float16 as a C/C++ source language type"

This is a recommit of r312781; in some build configurations
variable names are omitted, so changed the new regression
test accordingly.

llvm-svn: 312794
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index 5afaeef..685dc62 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -1866,6 +1866,7 @@
     Out << "$$T";
     break;
 
+  case BuiltinType::Float16:
   case BuiltinType::Float128:
   case BuiltinType::Half: {
     DiagnosticsEngine &Diags = Context.getDiags();