[LTO] Support Statistics

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 because ld64 does not destroy the LTOCodeGenerator.

Reviewers: dexonsmith, joker.eph

Subscribers: rafael, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D17302

llvm-svn: 261013