ada: Avoid null-exclusion checks for Node_Field_Table

ada: Avoid null-exclusion checks for Node_Field_Table

By generating the type of Node_Field_Table with a "not null" qualifier
we check the null exclusion of its elements only once, at the object
declaration.

Tiny performance improvement for the debug builds (because in production
builds checks are disabled anyway); semantics is unaffected.

gcc/ada/ChangeLog:

	* gen_il-gen.adb (Put_Tables): Add "not null" to the generated code.
	* rtsfind.adb (Cstring_Ptr): Same for table with predefined RE_Id
	error messages.
	* impunit.adb (Aunit_Record): Same for array of alternative unit names.