[Clang][Sema] Fix template name lookup for operator= (#90999)

This fixes a bug in #90152 where `operator=` was never looked up in the
current instantiation, resulting in `<` never being interpreted as the
start of a template argument list.

Since function templates are not copy/move assignment operators, the fix
is accomplished by allowing lookup in the current instantiation for
`operator=` when looking up a template name.