Change how DeclRef::toText works

[saipraveenb25]

We now ignore the decl-ref heirarchy since that only includes nodes with specialization info & simply walk up the tree of decls, while emitting any specializations present in the decl-ref.

Fixes: #5591

[csyonghe]

Instead of changing DeclRef::toText, which mess up all the tests, you can use ASTPrinter to get the full qualified name of a declref.

Call ASTPrinter::addOverridableDeclPath and then grab the content of the astprinter's string builder.

[saipraveenb25]

@csyonghe I fixed up the failing tests. (Only a couple of them were failing)
Further, one of those tests required this fix: tests/diagnostics/mismatching-types.slang was expecting full names for the nested types, but had the same problem that only a subset of the nodes were printed out.