[flang] Fix crashes due to failure to find a subprogram

In error recovery situations, the mappings from source locations
to scopes were failing in a way that tripped some asserts.
Specifically, FindPureProcedureContaining() wasn't coping well
when starting at the global scope.  (And since the global scope
no longer has a source range, clean up the Semantics constructor
to avoid confusion.)

Differential Revision: https://reviews.llvm.org/D103567