[flang] Catch procedure pointer interface error

A procedure pointer is allowed to name a specific intrinsic function
from F'2018 table 16.2 as its interface, but not other intrinsic
procedures.  Catch this error, and thereby also fix a crash resulting
from a failure later in compilation from failed characteristics;
while here, also catch the similar error with initializers.

Differential Revision: https://reviews.llvm.org/D103570