[flang] Fold IS_CONTIGUOUS() to .FALSE. when it is known to be

At present, IS_CONTIGUOUS() can only either fold to .TRUE. or
remain unknown.  The underlying analysis, however, is capable
of returning a tri-state result (true, false, or unknown).
Extend and expose it to folding so that IS_CONTIGUOUS() can
fold to .FALSE. as well as to .TRUE. when contiguity is
known.

Differential Revision: https://reviews.llvm.org/D134466