[demangler] Adjust unqualified name parsing

The unqualified name grammar includes <ctor-dtor-name>, but we handle
that specially in parseNestedName.  This is a little awkward.  We can
pass in the current scope and have parseUnqualifiedName deal with
cdtors too.  That also allows a couple of other simplifications:

1) parseUnqualifiedName can also build up the NestedName, when the
provided scope is non-null.  Which means ...

2) parseUnscopedName can pass a "std" scope in (and tailcall).

3) ... and also parseNestedName need not construct the nestedname itself.

4) also parseNestedName's detection of a cdtor-name doesn't have to
rule out a decomposition name anymore.

This change also makes adding module demangling more
straight-forwards, btw.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119154

GitOrigin-RevId: fbded4f42db1e52e3e0bc0fe2c7e3440ace455a4
1 file changed