[Ada] Reuse generic string hash for invocation signatures

[Ada] Reuse generic string hash for invocation signatures

Use a generic string hash routine for hashing of invocation signatures.
The System.String_Hash unit was added to GNAT in 2009 and shouldn't
cause any bootstrap problems these days. To be safe, we don't use the
GNAT.String_Hash renaming, which was added later.

Cleanup related to a new restriction No_Uninitialized_Local_Scalars,
which will also rely on GNAT hash tables. Semantics is unaffected.

gcc/ada/

	* ali.adb (Hash): Reuse GNAT.String_Hash.Hash and don't pollute
	the Name_Id table.