[flang] Relax two !DIR$ IGNORE_TKR error cases with descriptor arguments

Allow two currently erroneous cases of !DIR$ IGNORE_TKR errors: allocatable
and pointers, and IGNORE_TKR(R) on (other) arguments passed via descriptors.
Downgrade these cases to warnings when they appear in external interfaces,
since their implementations may well be in C.  But retain the error status
on these cases for module procedures, since the Fortran implementation
probably can't work.

Differential Revision: https://reviews.llvm.org/D148833