Fix internal error on extension with interface at -O2

Fix internal error on extension with interface at -O2

This is a regression present on the mainline, 10 and 9 branches, in the
form of an internal error with the Ada compiler when a covariant-only
thunk is inlined into its caller.

gcc/ada/
	* gcc-interface/trans.c (make_covariant_thunk): Set the DECL_CONTEXT
	of the parameters and do not set TREE_PUBLIC on the thunk.
	(maybe_make_gnu_thunk): Pass the alias to the covariant thunk.
	* gcc-interface/utils.c (finish_subprog_decl): Set the DECL_CONTEXT
	of the parameters here...
	(begin_subprog_body): ...instead of here.

gcc/testsuite/
	* gnat.dg/thunk2.adb, gnat.dg/thunk2.ads: New test.
	* gnat.dg/thunk2_pkg.ads: New helper.