[Ada] Small cleanup and improvement in inlining machinery

[Ada] Small cleanup and improvement in inlining machinery

This is a small cleanup in the inlining machinery of the front-end
dealing with back-end inlining.  It should save a few cycles at -O0 by
stopping it from doing useless work.  No functional changes.

2019-08-13  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch6.adb: Remove with and use clauses for Sem_Ch12.
	(Expand_Call_Helper): Swap the back-end inlining case and the
	special front-end expansion case.  In back-end inlining mode, do
	not invoke Add_Inlined_Body unless the call may be inlined.
	* inline.ads (Add_Pending_Instantiation): New function moved
	from...
	* inline.adb (Add_Inlined_Body): Simplify comment.  Turn test on
	the enclosing unit into assertion.
	(Add_Pending_Instantiation): New function moved from...
	* sem_ch12.ads (Add_Pending_Instantiation): ...here.
	* sem_ch12.adb (Add_Pending_Instantiation): ...here.

From-SVN: r274353