Add EP_CGSCCOptimizerLate extension point to PassManagerBuilder

The EP_CGSCCOptimizerLate extension point allows adding CallGraphSCC
passes at the end of the main CallGraphSCC passes and before any
function simplification passes run by CGPassManager.

Patch by Gor Nishanov!

Differential Revision: https://reviews.llvm.org/D22897

llvm-svn: 276953