[Ada] Remove repeated conversions between Source_Ptr and Int

[Ada] Remove repeated conversions between Source_Ptr and Int

Both Source_Ptr and Int are integer types (and even happen to have equal
ranges). Their values can be calculated without converting
back-and-forth, e.g.:

  Int (Loc1) - Int (Loc2)

can be written simply as:

  Int (Loc1 - Loc2)

Code cleanup related to handling of references to unset objects.
Offending occurrences found with various invocations of grep.

gcc/ada/

	* par-ch10.adb, scng.adb, sem_res.adb, sinfo-utils.adb,
	treepr.adb: Simplify calculations with Source_Ptr and Loc
	values.