[flang] Fix issue: Enforce specific intrinsic characteristics

Fix issue flang-compiler/f18#661.
The issue was that when probing a specific intrinsic, the
constraints of the related generic intrinsic were tested instead
of the more restrictive constraints of the specific intrinsic.

Original-commit: flang-compiler/f18@dd971e901d137f0cef9136250402521dbe266288
Reviewed-on: https://github.com/flang-compiler/f18/pull/680
Tree-same-pre-rewrite: false