[Ada] Compiler failure on an extended_return_statement in a block

[Ada] Compiler failure on an extended_return_statement in a block

When compiling with an assertion-enabled compiler, Assert_Failure can be
raised when expanded an extended_return_statement whose enclosing scope
is not a function (such as when it's a block_statement). The simple fix
is to change the Assert to test Current_Subprogram rather than Current_Scope.
Three such Assert pragmas are corrected in this way.

2018-07-31  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* exp_ch6.adb (Expand_N_Extended_Return_Statement): Replace
	calls to Current_Scope in three assertions with calls to
	Current_Subprogram.

gcc/testsuite/

	* gnat.dg/block_ext_return_assert_failure.adb: New testcase.

From-SVN: r263096