[PGO] Context sensitive PGO (part 2)

[PGO] Context sensitive PGO (part 2)

Part 2 of CSPGO changes (mostly related to ProfileSummary).
Note that I use a default parameter in setProfileSummary() and getSummary().
This is to break the dependency in clang. I will make the parameter explicit
after changing clang in a separated patch.

Differential Revision: https://reviews.llvm.org/D54175

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355131 91177308-0d34-0410-b5e6-96231b3b80d8