fortran: Honor array constructor type-spec during folding [PR107721]

fortran: Honor array constructor type-spec during folding [PR107721]

When an array constructor has an explicit type-spec, all elements must be
converted to that type and character elements must be padded/truncated to
the specified length.  This was working for simple cases but failing when:

1. Elements were parenthesized: [integer :: ([1.0])]
2. Constructors were nested: [[integer :: [1.0]]]
3. Character constructors were used with concatenation operators:
   [character(16) :: 'a', 'b'] // '|'
4. Nested character constructors with concatenation:
   [character(16) :: ['a', 'b']] // '|'
5. Outer constructor without type-spec wrapping inner with type-spec:
   [[character(16) :: ['a', 'b']]] // '|'
6. Nested character constructors with different type-specs:
   [character(16) :: [character(2) :: 'abcd']]

The root cause was twofold:

First, parenthesized expressions like ([1.0]) create EXPR_OP nodes that were
not being simplified before type conversion in check_constructor_type(),
so type conversion was applied to the EXPR_OP rather than its contents.

Second, character array constructors with explicit type-spec were not being
resolved before CONCAT operations in eval_intrinsic(), so elements retained
their original lengths instead of being padded to the type-spec length.
Additionally, nested array constructors needed their type-spec propagated
from the outer constructor.

The fix adds:
- Simplification of non-constant expressions in check_constructor_type()
  before attempting type conversion
- Call to gfc_check_constructor_type() in eval_intrinsic() to ensure
  type-spec conversion happens before any operations on array constructors
- Character array constructor resolution before CONCAT operations
- Recursive type-spec propagation for nested array constructors.
  When a nested array constructor has its own explicit type-spec, it is
  resolved first to enforce its own length (truncation/padding) before
  propagating the outer type-spec and resolving again.
- Detection of nested character constructors with explicit type-spec
  (via length_from_typespec) when the outer constructor has no type-spec

	PR fortran/107721
	PR fortran/102417

gcc/fortran/ChangeLog:

	* arith.cc (eval_intrinsic): Call gfc_check_constructor_type on
	array constructor operands with explicit type-spec to ensure
	element type conversion before operations.  Resolve character
	array constructors before CONCAT operations.
	(reduce_binary_ac, reduce_binary_ca, reduce_binary_aa): Preserve
	character length info in result arrays.
	* array.cc (check_constructor_type): Simplify non-constant
	expressions before type checking to handle parenthesized elements.
	Handle nested character array constructors with explicit type-spec
	when outer constructor has no type-spec.
	(gfc_resolve_character_array_constructor): Recursively propagate
	type-spec to nested array constructors.  If the nested constructor
	has an explicit type-spec, resolve it first before propagating
	the outer type-spec.

gcc/testsuite/ChangeLog:

	* gfortran.dg/array_constructor_typespec_1.f90: New test.

Co-authored-by: Harald Anlauf <anlauf@gcc.gnu.org>
Signed-off-by: Christopher Albert <albert@tugraz.at>