Reapply r231508 "CodeGen: Emit constant temporaries into read-only globals."

Reapply r231508 "CodeGen: Emit constant temporaries into read-only globals."

I disabled putting the new global into the same COMDAT as the function for now.
There's a fundamental problem when we inline references to the global but still
have the global in a COMDAT linked to the inlined function. Since this is only
an optimization there may be other versions of the COMDAT around that are
missing the new global and hell breaks loose at link time.

I hope the chromium build doesn't break this time :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231564 91177308-0d34-0410-b5e6-96231b3b80d8