[MS ABI] Correctly mangle classes without names for linkage purposes

[MS ABI] Correctly mangle classes without names for linkage purposes

A class without a name for linkage purposes gets a name along the lines
of <unnamed-type-foo> where foo is either the name of a declarator which
defined it (like a variable or field) or a
typedef-name (like a typedef or alias-declaration).

We handled the declarator case correctly but it would fall down during
template instantiation if the declarator didn't share the tag's type.
We failed to handle the typedef-name case at all.

Instead, keep track of the association between the two and keep it up to
date in the face of template instantiation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246469 91177308-0d34-0410-b5e6-96231b3b80d8