Add a new optimization option -Og 

Summary:
Just like gcc, we should have the -Og option as more and more software are using it:
https://llvm.org/bugs/show_bug.cgi?id=20765

Reviewers: echristo, dberlin, dblaikie, keith.walker.arm, rengolin

Subscribers: aprantl, friss, mehdi_amini, RKSimon, probinson, majnemer, cfe-commits

Differential Revision: https://reviews.llvm.org/D24998

llvm-svn: 286602
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 86a3c87..2bdd40c 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -98,6 +98,9 @@
     if (S == "s" || S == "z" || S.empty())
       return 2;
 
+    if (S == "g")
+      return 1;
+
     return getLastArgIntValue(Args, OPT_O, DefaultOpt, Diags);
   }