[flang] Apply definability checks in ALLOCATE/DEALLOCATE statements

The pointers and allocatables that appear in ALLOCATE and DEALLOCATE
statements need to be subject to the general definability checks so
that problems with e.g. PROTECTED objects can be caught.

(Also: regularize the capitalization of the DEALLOCATE error messages
while I'm in here so that they're consistent with the messages that
can come out for ALLOCATE.)

Differential Revision: https://reviews.llvm.org/D140149