add a tool to generate symbol remapping files.

Summary:
The new tool llvm-cxxmap builds a symbol mapping table from a file containing
a description of partial equivalences to apply to mangled names and files
containing old and new symbol tables.

Reviewers: davidxl

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D51470

llvm-svn: 342168