This adds initial support for a demangling library (LLVMDemangle)
and tool (llvm-undname) for demangling Microsoft names. This
doesn't cover 100% of cases and there are some known limitations
which I intend to address in followup patches, at least until such
time that we have (near) 100% test coverage matching up with all
of the test cases in clang/test/CodeGenCXX/mangle-ms-*.
Differential Revision: https://reviews.llvm.org/D49552
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337584 91177308-0d34-0410-b5e6-96231b3b80d8