ada: Tune hash function for cross-reference entries

Tune the hash function that combines entity identifiers with source
locations of where those entities are referenced. Previously the source
location was multiplied by 2 ** 7 (i.e. shifted left by 7 bits), then
added to the entity identifier, and finally divided modulo 2 ** 16 (i.e.
masked to only use the lowest 16 bits). This hash routine caused
collisions that could make some tests up to twice slower.

With a large entity number the source location was only contributing few
bits to the hash value. This large entity number might correspond to
entity like Ada.Characters.Latin_1.NUL that occurs thousands of times in
generated code.

gcc/ada/

	* lib-xref.adb (Hash): Tune hash function.

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