Fortran: improve expansion of constant array expressions within constructors

Fortran: improve expansion of constant array expressions within constructors

gcc/fortran/ChangeLog:

	PR fortran/102787
	* array.c (expand_constructor): When encountering a constant array
	expression or array section within a constructor, simplify it to
	enable better expansion.

gcc/testsuite/ChangeLog:

	* gfortran.dg/array_constructor_54.f90: New test.

(cherry picked from commit 6b8ecbc6d6652d061d7c72c64352d51eca2df6ca)