Add a --check-graph option to llvmc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61989 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CommandGuide/llvmc.pod b/docs/CommandGuide/llvmc.pod
index 7bfc3d7..17d85d7 100644
--- a/docs/CommandGuide/llvmc.pod
+++ b/docs/CommandGuide/llvmc.pod
@@ -42,6 +42,12 @@
 
 Enable verbose mode, i.e. print out all executed commands.
 
+=item B<--check-graph>
+
+Check the compilation for common errors like mismatched output/input
+language names, multiple default edges and cycles. Hidden option,
+useful for debugging.
+
 =item B<--view-graph>
 
 Show a graphical representation of the compilation graph. Requires