[flang] Better error recovery for bad submodules

When a submodule appears in a source file and the compiler can't find the
named ancestor module (and submodule, if one appears), crashes may occur
later due to the absence of a scope.  For better resilience, a dummy
ancestral scope should be generated within which the submodule scope
can be created.

Differential Revision: https://reviews.llvm.org/D128760