[Polly] Fix memory leak in DependenceAnalysis::Result::abandonDepende… (#211514)

abandonDependences() uses unique_ptr::release() which releases
ownership without freeing the Dependences object, causing a memory
leak. Use unique_ptr::reset() instead to properly delete the object
before nullifying the pointer.

The issue was found when AddressSanitizer is enabled in the build.

GitOrigin-RevId: a65df8a1d42a34217bba1354c064b3fa40529507