Respect a comment in sinfo.ads, which says: "Unchecked type conversion
nodes should be created by calling Tbuild.Unchecked_Convert_To, rather
than by directly calling Nmake.Make_Unchecked_Type_Conversion."

No test appears to be affected by this change, so this is just a
cleanup.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch6.adb (Build_Static_Check_Helper_Call): Replace explicit
	call to Make_Unchecked_Type_Conversion with a call to
	Unchecked_Convert_To.
	* tbuild.adb (Unchecked_Convert_To): Fix whitespace.