fortran: ICE or wrong-code for ASSOCIATE selector that is a type-bound user-defined operator

Three related bugs prevented ASSOCIATE selectors that are type-bound
user-defined operator expressions from compiling correctly.

Bug 1 (class.cc): find_typebound_proc_uop returned NULL immediately when
the derived type has no f2k_derived namespace, bypassing the parent-type
inheritance walk.  This caused inherited UDOs to be silently not found.
Fix: set root = NULL and let the loop reach the parent type instead.

Bug 2 (resolve.cc): resolve_typebound_procedures called resolve_symbol on
the parent type only after an early return that fires when the derived type
has no direct type-bound bindings.  This left parent-type bindings
unresolved when searched via gfc_find_typebound_user_op.
Fix: move resolve_symbol(super_type) before the early return.

Bug 3 (match.cc): match_association_list did not handle ASSOCIATE selectors
of the form .uop. expr or the nested case .uop2. (.uop1. expr).  When the
selector's type was BT_UNKNOWN at parse time the name of the associate
variable was left untyped, producing a "Syntax error in expression" ICE in
the body of the ASSOCIATE construct.

Add three helpers before match_association_list:
  - resolve_assoc_operand: attempts gfc_resolve_expr on EXPR_FUNCTION
    operands and falls back to gfc_find_dt_in_generic for constructor calls
    whose argument types are not yet known.
  - infer_typebound_uop_type: reads the return type of a type-bound UDO
    directly from specific_st->n.tb->u.specific->n.sym without calling
    gfc_resolve_symbol, avoiding a resolve_symbol_called race condition.
  - extend_assoc_op: walks the expression tree bottom-up, propagating
    types through INTRINSIC_PARENTHESES wrappers before calling the two
    helpers above on each INTRINSIC_USER node.

When the selector is an INTRINSIC_USER EXPR_OP with BT_UNKNOWN type,
call extend_assoc_op on the operands, then gfc_extend_expr (errors
suppressed).  Accept the result when gfc_extend_expr returns MATCH_YES or
when it returns MATCH_ERROR but has already converted the node to
EXPR_COMPCALL with a known type (the full resolution pass finishes it).

Assisted-by: Claude Sonnet 4.6

	PR fortran/125528

gcc/fortran/ChangeLog:

	* class.cc (find_typebound_proc_uop): Set root = NULL instead of
	returning NULL when derived type lacks f2k_derived, so parent-type
	type-bound procedures and operators are still found via inheritance.
	* match.cc (resolve_assoc_operand): New helper.
	(infer_typebound_uop_type): New helper.
	(extend_assoc_op): New helper.
	(match_association_list): Handle ASSOCIATE selectors that are
	type-bound user-defined operator expressions, including nested cases.
	* resolve.cc (resolve_typebound_procedures): Move resolve_symbol
	call for the parent type before the early return so inherited
	type-bound bindings are resolved even when the child type has none
	of its own.

gcc/testsuite/ChangeLog:

	* gfortran.dg/associate_80.f90: New test.

(cherry picked from commit 6340639219bb2d1816c94e5de76ec3642453bcb2)