[Demangle] Add minimal support for D programming language

This patch adds minimal support for D programming language demangling on LLVM
core based on the D name mangling spec. This will allow easier integration on a
future LLDB plugin for D either in the upstream tree or outside of it.

Minimal support includes recognizing D demangling encoding and at least one
mangling name, which in this case is `_Dmain` mangle.

Reviewed By: jhenderson, lattner

Differential Revision: https://reviews.llvm.org/D111414