Objective-C : Implement SEL as a built-in typedef.

Objective-C : Implement SEL as a built-in typedef.

The reference implementation for Objective-C provides the SEL
typedef (although it is also available from <objc/objc.h>).

gcc/objc/ChangeLog:

	* objc-act.c (synth_module_prologue): Get the SEL identifier.
	* objc-act.h (enum objc_tree_index): Add OCTI_SEL_NAME.
	(objc_selector_name): New.
	(SEL_TYPEDEF_NAME): New.
	* objc-gnu-runtime-abi-01.c
	(gnu_runtime_01_initialize): Initialize SEL typedef.
	* objc-next-runtime-abi-01.c
	(next_runtime_01_initialize): Likewise.
	* objc-next-runtime-abi-02.c

gcc/testsuite/ChangeLog:

	* obj-c++.dg/SEL-typedef.mm: New test.
	* objc.dg/SEL-typedef.m: New test.