[ObjC] Recognise 'instancetype' as equivalent to 'id'.

[ObjC] Recognise 'instancetype' as equivalent to 'id'.

This is primarily about fixing a number of testsuite fails, it
implements the parsing of 'instancetype' but doesn't alter the
code gen (relative to using id in the same position). It is
part of the work-arounds for PR90709.

gcc/objc/

2019-09-05  Iain Sandoe  <iain@sandoe.co.uk>

	Backport from mainline.
	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/

2019-09-05  Iain Sandoe  <iain@sandoe.co.uk>

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

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

From-SVN: r275428