Make getDISubprogram(const Function *F) available in LLVM

Reviewed at http://reviews.llvm.org/D5950

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220536 91177308-0d34-0410-b5e6-96231b3b80d8
If requested, apply function merging at -O0 too. It's useful there to reduce the time to compile.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220537 91177308-0d34-0410-b5e6-96231b3b80d8