WholeProgramDevirt: generate more detailed and accurate remarks.

Summary:
Keep track of all methods for which we have devirtualized at least
one call and then print them sorted alphabetically. That allows to
avoid duplicates and also makes the order deterministic.

Add optimization names into the remarks, so that it's easier to
understand how has each method been devirtualized.

Fix a bug when wrong methods could have been reported for
tryVirtualConstProp.

Reviewers: kcc, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23297

llvm-svn: 278389