[Ada] Fix bogus compilation error with Elaborate_Body and -gnatN

[Ada] Fix bogus compilation error with Elaborate_Body and -gnatN

This fixes a bogus compilation error when a unit with SPARK_Mode
containing a pragma Elaborate_Body is with-ed by a generic unit
containing an inlined subprogram, and front-end inlining is enabled.

2019-08-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_prag.adb (Is_Before_First_Decl): Deal with rewritten
	pragmas.

gcc/testsuite/

	* gnat.dg/elab8.adb, gnat.dg/elab8_gen.adb,
	gnat.dg/elab8_gen.ads, gnat.dg/elab8_pkg.adb,
	gnat.dg/elab8_pkg.ads: New testcase.

From-SVN: r274664