Fortran: Fix reallocation on assignment for kind=4 strings [PR107508]

Fortran: Fix reallocation on assignment for kind=4 strings [PR107508]

The check whether reallocation on assignment was required did not handle
kind=4 characters correctly such that there was always a reallocation,
implying issues with pointer addresses and lower bounds.  Additionally,
with all deferred strings, the old memory was not freed on reallocation.
And, finally, inside the block which was only executed if string lengths
or bounds or dynamic types changed, was a subcheck of the same, which
was effectively a no op but still confusing and at least added with -O0
extra instructions to the binary.

	PR fortran/107508

gcc/fortran/ChangeLog:

	* trans-array.cc (gfc_alloc_allocatable_for_assignment): Fix
	string-length check, plug memory leak, and avoid generation of
	effectively no-op code.
	* trans-expr.cc (alloc_scalar_allocatable_for_assignment): Extend
	comment; minor cleanup.

gcc/testsuite/ChangeLog:

	* gfortran.dg/widechar_11.f90: New test.