ada: Clean up unnecesary call in resolution of overloaded expressions

ada: Clean up unnecesary call in resolution of overloaded expressions

When experimentally enabling frontend inlining by default, the
unnecessary call to Comes_From_Predefined_Lib_Unit in Resolve appears to
be a performance bottleneck (most likely this call is expensive because
it involves a loop over the currently inlined subprograms).

Code cleanup; semantics is unaffected.

gcc/ada/

	* sem_res.adb (Resolve): Only call Comes_From_Predefined_Lib_Unit
	when its result might be needed.