[demangler] Add a partial demangling API for LLDB.

This parses a mangled name into an AST (typically an intermediate stage in
itaniumDemangle) and provides some functions to query certain properties or
print certain parts of the demangled name.

Differential revision: https://reviews.llvm.org/D44668

llvm-svn: 329951
GitOrigin-RevId: 67d82d6eddf2369369a88acb86560657becf9218