[MS Demangler] Expose the Demangler AST publicly.

LLDB would like to use this in order to build a clang AST from
a mangled name.

This is NFC otherwise.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345837 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed