Fortran: Implement F2018 IMPORT statements [PR106135]

Fortran:  Implement F2018 IMPORT statements [PR106135]

2025-09-09  Paul Thomas  <pault@gcc.gnu.org>

gcc/fortran
	PR fortran/106135
	* decl.cc (build_sym): Emit an error if a symbol associated by
	an IMPORT, ONLY or IMPORT, all statement is being redeclared.
	(gfc_match_import): Parse and check the F2018 versions of the
	IMPORT statement. For scopes other than and interface body, if
	the symbol cannot be found in the host scope, generate it and
	set it up such that gfc_fixup_sibling_symbols can transfer its
	'imported attribute' if it turnes out to be a not yet parsed
	procedure. Test for violations of C897-8100.
	* gfortran.h : Add 'import_only' to the gfc_symtree structure.
	Add the enum, 'importstate', which is used for values the new
	field 'import_state' in gfc_namespace.
	* parse.cc (gfc_fixup_sibling_symbols): Transfer the attribute
	'imported' to the new symbol.
	* resolve.cc (check_sym_import_status, check_import_status):
	New functions to test symbols and expressions for violations of
	F2018:C8102.
	(resolve_call): Test the 'resolved_sym' against C8102 by a call
	to 'check_sym_import_status'.
	(gfc_resolve_expr): If the expression is OK and an IMPORT
	statement has been registered in the current scope, test C102
	by calling 'check_import_status'.
	(resolve_select_type): Test the declared derived type in TYPE
	IS and CLASS IS statements.

gcc/testsuite/
	PR fortran/106135
	* gfortran.dg/import3.f90: Use -std=f2008 and comment on change
	in error message texts with f2018.
	* gfortran.dg/import12.f90: New test.