Fix misoptimization at -O2 in LTO mode

Fix misoptimization at -O2 in LTO mode

This is a regression in recent releases. The problem is that the IPA mod/ref
pass looks through the (nominal) type of a pointer-to-discriminated-type
parameter in a call to a subprogram in order to see the (actual) type used
for the dereferences of the parameter in the callee, which is a
pointer-to-constrained-subtype.

Historically the discriminated type is marked with the may_alias attribute
because of the symmetric effect for the argument in the caller, so we mark
the constrained subtype with the attribute now for the sake of the callee.

gcc/ada/
	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Record_Subtype>: Set
	the may_alias attribute if a specific GCC type is built.