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/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index c715449..8aade1a 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -226,7 +226,7 @@
 Code Generation Options
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-.. option:: -O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -O, -O4
+.. option:: -O0, -O1, -O2, -O3, -Ofast, -Os, -Oz, -Og, -O, -O4
 
   Specify which optimization level to use:
 
@@ -252,6 +252,9 @@
     :option:`-Oz` Like :option:`-Os` (and thus :option:`-O2`), but reduces code
     size further.
 
+    :option:`-Og` Like :option:`-O1`. In future versions, this option might 
+    disable different optimizations in order to improve debuggability.
+
     :option:`-O` Equivalent to :option:`-O2`.
 
     :option:`-O4` and higher