[flang] Add mapping of source location to Scope

Each Scope now tracks the source locations that it and its nested scopes
span. This is achieved by extending the source range of a scope for each
statement encountered while it is the current scope.

Semantics::FindScope maps a source location (from the cooked character
stream) to the narrowest scope that contains it.

Original-commit: flang-compiler/f18@7b4d2bb113e4a1018c5f6aa740b02e423ff61799
Reviewed-on: https://github.com/flang-compiler/f18/pull/230
Tree-same-pre-rewrite: false