Add data structure to form equivalence classes of mangled names.

Summary:
Given a set of equivalent name fragments, this mechanism determines whether two
mangled names are equivalent. The intent is to use this for fuzzy matching of
profile data against the program after certain refactorings are performed.

Reviewers: erik.pilkington, dlj

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D50935

llvm-svn: 340663