[LTO] Support Statistics

[anemet]

Summary

I thought -Xlinker -mllvm -Xlinker -stats worked at some point but maybe
it never did.

For clang, I believe that stats are printed from cc1_main. This patch
also prints them for LTO, specifically right after codegen happens.

I only looked at the C API for LTO briefly to see if this is a good
place. Probably there are still cases where this wouldn't be printed
but it seems to be working for the common case. I also experimented
putting this in the LTOCodeGenerator destructor but that didn't trigger
for me.

[mehdi_amini]

LGTM.

(Note that ld64 is not destroying the LTOCodeGenerator)

[• rafael]

Can you add a test to test/tools/lto?

[anemet]

Good idea, will add a test.

lib/LTO/LTOCodeGenerator.cpp

 //===-LTOCodeGenerator.cpp - LLVM Link Time Optimizer ---------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the Link Time Optimization library. This library is
 // intended to be used by linker to optimize code at link time.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/LTO/LTOCodeGenerator.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Analysis/Passes.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Bitcode/ReaderWriter.h"
 #include "llvm/CodeGen/ParallelCG.h"
 #include "llvm/CodeGen/RuntimeLibcalls.h"
 #include "llvm/Config/config.h"
bool LTOCodeGenerator::compileOptimized(ArrayRef<raw_pwrite_stream *> Out) {
   // parallelism level 1. This is achieved by having splitCodeGen return the
   // original module at parallelism level 1 which we then assign back to
   // MergedModule.
   MergedModule =
       splitCodeGen(std::move(MergedModule), Out, MCpu, FeatureStr, Options,
                    RelocModel, CodeModel::Default, CGOptLevel, FileType,
                    ShouldRestoreGlobalsLinkage);
 
+  // If statistics were requested, print them out after codegen.
+  if (llvm::AreStatisticsEnabled())
+    llvm::PrintStatistics();
+
   return true;
 }
 
 /// setCodeGenDebugOptions - Set codegen debugging options to aid in debugging
 /// LTO problems.
 void LTOCodeGenerator::setCodeGenDebugOptions(const char *Options) {
   for (std::pair<StringRef, StringRef> o = getToken(Options); !o.first.empty();
        o = getToken(o.second))