fortran: Implement optional type spec for DO CONCURRENT [PR96255]

fortran: Implement optional type spec for DO CONCURRENT [PR96255]

This patch adds support for the F2008 optional integer type specification
in DO CONCURRENT and FORALL headers, allowing constructs like:

  do concurrent (integer :: i=1:10)

The implementation handles type spec matching, creates shadow variables
when the type spec differs from any outer scope variable, and converts
iterator expressions to match the specified type.

Shadow variable implementation:
When a type-spec is provided and differs from an outer scope variable,
a shadow variable with the specified type is created (with _ prefix).
A recursive expression walker substitutes all references to the outer
variable with the shadow variable throughout the DO CONCURRENT body,
including in array subscripts, substrings, and nested operations.

Constraint enforcement:
Sets gfc_do_concurrent_flag properly (1 for block context, 2 for mask
context) to enable F2008 C1139 enforcement, ensuring only PURE procedures
are allowed in DO CONCURRENT constructs.

Additional fixes:
- Extract apply_typespec_to_iterator() helper to eliminate duplicated
  shadow variable creation code (~70 lines)
- Add NULL pointer checks for shadow variables
- Fix iterator counting to handle both EXEC_FORALL and EXEC_DO_CONCURRENT
- Skip FORALL obsolescence warning for DO CONCURRENT (F2018)
- Suppress many-to-one assignment warning for DO CONCURRENT (reductions
  are valid, formalized with REDUCE locality-spec in F2023)

	PR fortran/96255

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_forall_iterator): Add bool shadow field.
	* match.cc (apply_typespec_to_iterator): New helper function to
	consolidate shadow variable creation logic.
	(match_forall_header): Add type-spec parsing for DO CONCURRENT
	and FORALL. Create shadow variables when type-spec differs from
	outer scope. Replace duplicated code with apply_typespec_to_iterator.
	* resolve.cc (replace_in_expr_recursive): New function to recursively
	walk expressions and replace symbol references.
	(replace_in_code_recursive): New function to recursively walk code
	blocks and replace symbol references.
	(gfc_replace_forall_variable): New entry point for shadow variable
	substitution.
	(gfc_resolve_assign_in_forall): Skip many-to-one assignment warning
	for DO CONCURRENT.
	(gfc_count_forall_iterators): Handle both EXEC_FORALL and
	EXEC_DO_CONCURRENT with assertion.
	(gfc_resolve_forall): Skip F2018 obsolescence warning for DO
	CONCURRENT. Fix memory allocation check. Add NULL checks for shadow
	variables. Implement shadow variable walker.
	(gfc_resolve_code): Set gfc_do_concurrent_flag for DO CONCURRENT
	constructs to enable constraint checking.

gcc/testsuite/ChangeLog:

	* gfortran.dg/do_concurrent_typespec_1.f90: New test covering all
	shadowing scenarios: undeclared variable, same kind shadowing, and
	different kind shadowing.

Co-authored-by: Steve Kargl <kargl@gcc.gnu.org>
Co-authored-by: Jerry DeLisle <jvdelisle@gcc.gnu.org>
Signed-off-by: Christopher Albert <albert@tugraz.at>