[flang] Catch more bad TARGET= arguments to ASSOCIATED()

The TARGET= argument to the intrinsic function ASSOCIATED() must be
a valid target for the POINTER= argument, but we are missing some
cases, such as parenthesized expressions.  Add more checking, and
restructure the logic a bit to make the case analysis structure
more clear.

Fixes llvm-test-suite/Fortran/gfortran/regression/associated_target_1.f90.

Differential Revision: https://reviews.llvm.org/D157341