[flang] Retrieve the correct scope when lowering SELECT TYPE

Scope to retrieve the associating entity is needed to map the
symbol to the IR value. The scope can be found with a source
information. For the type case in SELECT TYPE construct, the source
information is on the Statement<TypeCase>. This patch updates
the lowering so the scopes for each type guards is retrieved
before the processing.

Reviewed By: PeteSteinfeld, vdonaldson

Differential Revision: https://reviews.llvm.org/D144133