[demangler][NFC] Use def file for node names

In order to add a unit test, we need to expose the node names beyond
ItaniumDemangle.h.  This breaks them out into a def file.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D122739

GitOrigin-RevId: 369337e3c2225e3a09f62a6016a93752aa7e3812
3 files changed