objective-c - add instancetype.

objective-c - add instancetype.

The instancetype has been added as a typedef alias to id
in order to allow diagnosis of cases where a class is used
or returned where an instance is expected.

This adds the typedef, and tests that we can parse it.
It doesn't alter the diagnostics yet.

gcc/objc/

2019-05-18  Iain Sandoe  <iain@sandoe.co.uk>

	* objc/objc-act.h (OCTI_INSTANCE_TYPE, OCTI_INSTANCETYPE_NAME): New.
	(objc_global_trees): Add instance type and name.
	(INSTANCE_TYPEDEF_NAME): New.
	* objc/objc-act.c (synth_module_prologue): Build decls for
	objc_instancetype_type and objc_instancetype_name.

gcc/testsuite/

2019-05-18  Iain Sandoe  <iain@sandoe.co.uk>

	* objc.dg/instancetype-0.m: New.

From-SVN: r271370