[lldb] Fix simple template names and template params with scope qualifiers

Followup to D134378.

With PrintingPolicy::SuppressScope, we'd also not print the scope in template params. The intention was only to skip the scope for the class because we expect template params to be fully qualified when comparing them for simple template names.

Instead, use `NamedDecl::getNameForDiagnostic` if we're dealing with a tag, which is what we actually use when emitting debug info in clang. That already has an option to suppress the scope on the base name.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D137583