[flang] Catch (and fix) attempts to create an invalid source range for a Scope

When Scope::AddSourceRange() is called to extend the scope's source range to
include another snippet from a cooked character stream, add a check to ensure
that the new range is part of the same cooked character stream as the rest
of the scope.

And fix the bug that was causing such invalid source ranges to be created:
a submodule's Scope is a children of its parent's in the Scope tree, but
it may or may not be part of the same source file, and it is certainly
not enclosed in the parent's source range.  So don't propagate Scope
source range expansion from a submodule to its parent.

Differential Revision: https://reviews.llvm.org/D150714