[flang] Add nested scoping to label distinctness checking

Fortran defines derived type definitions and explicit interface
blocks for subroutines and functions to be nestable scopes for
statement labels, even though such labels are useless for all
purposes.  Handle these scopes in label resolution so that bogus
errors about conflicting labels in "real" code don't come out.

Note that BLOCK constructs could have also been defined as scopes
for statement labeling, but were not.

Differential Revision: https://reviews.llvm.org/D132679