[flang] Tune handling of LEN type parameter discrepancies on ALLOCATE

Presently, semantics doesn't check for discrepancies between known
constant corresponding LEN type parameters between the declared type
of an allocatable/pointer and either the type-spec or the SOURCE=/MOLD=
on an ALLOCATE statement.

This allows discrepancies between character lengths to go unchecked.
Some compilers accept mismatched character lengths on SOURCE=/MOLD=
and the allocate object, and that's useful and unambiguous feature
that already works in f18 via truncation or padding.  A portability
warning should issue, however.

But for mismatched character lengths between an allocate object and
an explicit type-spec, and for any mismatch between derived type
LEN type parameters, an error is appropriate.

Differential Revision: https://reviews.llvm.org/D146583