Fix wrong assignment to mutable Out parameter of task entry

Fix wrong assignment to mutable Out parameter of task entry

Under very specific circumstances the compiler can generate a wrong
assignment to a mutable record object which contains an array component,
because it does not correctly handle the update of the discriminant.

gcc/ada/ChangeLog
	* gcc-interface/gigi.h (operand_type): New static inline function.
	* gcc-interface/trans.c (gnat_to_gnu): Do not suppress conversion
	to the resulty type at the end for array types.
	* gcc-interface/utils2.c (build_binary_op) <MODIFY_EXPR>: Do not
	remove conversions between array types on the LHS.

gcc/testsuite/ChangeLog
	* gnat.dg/array39.adb: New test.
	* gnat.dg/array39_pkg.ads: New helper.
	* gnat.dg/array39_pkg.adb: Likewise.