[flang] Fix folding of LEN(f(...))

LEN(f(...)), where "f" is a non-intrinsic function, should not be folded
to anything else unless the result is a known constant value.  While there
are conceivable cases in which we could do better (e.g., an internal function
whose length is a host-associated INTENT(IN) dummy argument), there are
other cases that we're getting wrong.

Differential Revision: https://reviews.llvm.org/D128759