[Ada] Fix spurious error on derived record passed as Out parameter

[Ada] Fix spurious error on derived record passed as Out parameter

This fixlet gets rid of a spurious error issued in the specific case of
a call to a subprogram taking an Out parameter of a discriminated record
type without default discriminants, if the actual parameter is the
result of the conversion to the record type of a variable whose type is
derived from the record and has a representation clause.

The compiler was failing to initialize the temporary made around the
call because of the representation clause, but this is required for a
type with discriminants because discriminants may be read by the called
subprogram.

2018-10-09  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch6.adb (Add_Call_By_Copy_Code): Initialize the temporary
	made for an Out parameter if the formal type has discriminants.

gcc/testsuite/

	* gnat.dg/derived_type5.adb, gnat.dg/derived_type5_pkg.ads: New
	testcase.

From-SVN: r264980