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/Format/FormatToken.cpp b/clang/lib/Format/FormatToken.cpp
index ba5bf03a..d1f16eb 100644
--- a/clang/lib/Format/FormatToken.cpp
+++ b/clang/lib/Format/FormatToken.cpp
@@ -52,6 +52,7 @@
   case tok::kw_half:
   case tok::kw_float:
   case tok::kw_double:
+  case tok::kw__Float16:
   case tok::kw___float128:
   case tok::kw_wchar_t:
   case tok::kw_bool: