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/Index/USRGeneration.cpp b/clang/lib/Index/USRGeneration.cpp
index c4fe737..7afbb8b 100644
--- a/clang/lib/Index/USRGeneration.cpp
+++ b/clang/lib/Index/USRGeneration.cpp
@@ -680,6 +680,7 @@
           c = 'K'; break;
         case BuiltinType::Int128:
           c = 'J'; break;
+        case BuiltinType::Float16:
         case BuiltinType::Half:
           c = 'h'; break;
         case BuiltinType::Float: