[Ada] Fix type mismatch in extended return statement expansion

[Ada] Fix type mismatch in extended return statement expansion

This fixes a (sub)type mismatch in the expansion of an extended return
statement generated for a built-in-place function that doesn't need a
BIP_Alloc_Form parameter but returns unconstrained.

No functional changes.

2019-08-21  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch6.adb (Expand_N_Extended_Return_Statement): In the case
	of a built-in-place function that doesn't need a BIP_Alloc_Form
	parameter but returns unconstrained, build the return
	consistently using the function's result subtype.  Remove bypass
	added in previous change.

From-SVN: r274782