[fir][NFC] rename canBePointerOrHeapElementType to cannotBePointerOrHeapElementType

This patch is part of the upstreaming effort from fir-dev branch.
Rename the function so the name conveys better what it does.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D110323

Co-authored-by: schweitz
Co-authored-by: jeanPerier