[flang] Handle undeclared names in EQUIVALENCE statements

Names in EQUIVALENCE statements are only allowed to indicate local
objects as per 19.5.1.4, paragraph 2, item (10).  Thus, a name appearing
in an EQUIVALENCE statement with no corresponding declaration in the
same scope is an implicit declaration of the name.  If that scope
contains an IMPLICIT NONE, it's an error.

I implemented this by adding a state variable to ScopeHandler to
indicate if we're resolving the names in an EQUIVALENCE statement and
then checked this state when resolving names.  I also added a test to
the existing tests for EQUIVALENCE statements.

Differential Revision: https://reviews.llvm.org/D93345