[Ada] Fix spurious error on nested instantiation with inlining

[Ada] Fix spurious error on nested instantiation with inlining

This prevents the compiler from issuing a spurious error in a convoluted
case where a child generic package declared in an enclosing parent
generic package, which contains a second child generic package, contains
an inlined subprogram and the second child generic package contains an
instantiation of the first, and the enclosing parent generic package is
instantiated with inlining across units enabled (-gnatn[12]).

The problem is that the compiler attempts to instantiate the body of the
first child generic package in the context of the enclosing parent
generic package, instead of doing it in the context of the instantiation
of the parent generic package, because of the presence of the inlined
subprogram.

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

gcc/ada/

	* exp_ch6.adb (Expand_Call_Helper): If back-end inlining is
	enabled, also instantiate the body of a generic unit containing
	a subprogram subject to aspect/pragma Inline_Always at
	optimization level zero.
	* sem_ch12.adb (Might_Inline_Subp): Minor tweak.
	(Analyze_Package_Instantiation): Do not instantiate the package
	body because of inlining considerations if the instantiation is
	done in a generic unit.  Move around similar condition involving
	the main unit.  Add test on Back_End_Inlining to processing for
	front-end inlining.

gcc/testsuite/

	* gnat.dg/generic_inst8.adb, gnat.dg/generic_inst8.ads,
	gnat.dg/generic_inst8_g.adb, gnat.dg/generic_inst8_g.ads: New
	testcase.

From-SVN: r274336