[flang] Fix scope in which undeclared symbols are created

Don't create new symbols in FORALL, implied DO, or other
construct scopes when an undeclared name appears; use the
innermost enclosing program unit's scope.  This clears up
a pending TODO in name resolution, and also exposes (& fixes)
an unnoticed name resolution problem in a module file test.

Differential Revision: https://reviews.llvm.org/D109095