Options: Use AliasArgs for -O -> -O2 translation.

This way we can do the translation in the .td file rather than manually.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187583 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index bc003fa..20dbaad 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -2705,14 +2705,10 @@
   // preprocessed inputs and configure concludes that -fPIC is not supported.
   Args.ClaimAllArgs(options::OPT_D);
 
-  // Manually translate -O to -O2 and -O4 to -O3; let clang reject
-  // others.
+  // Manually translate -O4 to -O3; let clang reject others.
   if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
     if (A->getOption().matches(options::OPT_O4))
       CmdArgs.push_back("-O3");
-    else if (A->getOption().matches(options::OPT_O) &&
-             A->getValue()[0] == '\0')
-      CmdArgs.push_back("-O2");
     else
       A->render(Args, CmdArgs);
   }