[demangler] Add support for C++20 modules

Add support for module name demangling.  We have two new demangler
nodes -- ModuleName and ModuleEntity. The former represents a module
name in a hierarchical fashion. The latter is the combination of a
(name) node and a module name. Because module names and entity
identities use the same substitution encoding, we have to adjust the
flow of how substitutions are handled, and examine the substituted
node to know how to deal with it.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D119933

GitOrigin-RevId: c354167ae217f90399bff9a644fffb3e9a6b4334
2 files changed