Use an option alias to implement -gmlt

Review feedback from Reid Kleckner on r203603.

llvm-svn: 204755
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index c90fb81..ea60551 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -2695,8 +2695,7 @@
   // are preserved, all other debug options are substituted with "-g".
   Args.ClaimAllArgs(options::OPT_g_Group);
   if (Arg *A = Args.getLastArg(options::OPT_g_Group)) {
-    if (A->getOption().matches(options::OPT_gline_tables_only) ||
-        A->getOption().matches(options::OPT_gmlt)) {
+    if (A->getOption().matches(options::OPT_gline_tables_only)) {
       // FIXME: we should support specifying dwarf version with
       // -gline-tables-only.
       CmdArgs.push_back("-gline-tables-only");