Fix artificial overflow during GENERIC folding

Fix artificial overflow during GENERIC folding

The Ada compiler gives a bogus warning:
storage_offset1.ads:16:52: warning: Constraint_Error will be raised at run
time [enabled by default]

Ironically enough, this occurs because of an intermediate conversion to an
unsigned type which is supposed to hide overflows but is counter-productive
for constants because TREE_OVERFLOW is always set for them, so it ends up
setting a bogus TREE_OVERFLOW when converting back to the original type.

The fix simply redirects INTEGER_CSTs to the other, direct path without the
intermediate conversion to the unsigned type.

gcc/
	* match.pd ((T)P - (T)(P + A) -> -(T) A): Avoid artificial overflow
	on constants.

gcc/testsuite/
	* gnat.dg/specs/storage_offset1.ads: New test.