Implement mcount profiling, enabled via -pg.

llvm-svn: 125282
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 3108477..2103856 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -948,6 +948,7 @@
   Opts.VerifyModule = !Args.hasArg(OPT_disable_llvm_verifier);
 
   Opts.InstrumentFunctions = Args.hasArg(OPT_finstrument_functions);
+  Opts.InstrumentForProfiling = Args.hasArg(OPT_pg);
 
   if (Arg *A = Args.getLastArg(OPT_fobjc_dispatch_method_EQ)) {
     llvm::StringRef Name = A->getValue(Args);