[LLVMDemangle] Move some utility classes to header files.

In a followup I'm looking to add a Microsoft demangler.  Doing
so needs a lot of the same utility classes and feature test
macros which are already implemented in ItaniumDemangle.cpp.
So move all of these things into header files so that they
can be re-used by a new demangler.

Differential Revision: https://reviews.llvm.org/D49399

llvm-svn: 337217