analyzer: add test coverage for gfortran ICE fix [PR 93779]

PR analyzer/93779 reports an ICE in gfortran with -fanalyzer
that was fixed for GCC 10 by a workaround in
f76a88eb; the tree code in
question is a FUNCTION_DECL.

Given that I want to rework the above patch at some point, it seems
prudent to add test coverage to ensure the ICE doesn't come back,
which this patch does.

gcc/testsuite/ChangeLog:
	PR analyzer/93779
	* gfortran.dg/analyzer/pr88304-2.f90: New test, adapted from
	gfortran.fortran-torture/compile/pr88304-2.f90