[SSAF] Remove UnsafeBufferUsageTest.h (#195201)
The idea is that we do not want to have test-only functionality in any
interface.
We have lit tests
(clang/test/Analysis/Scalable/UnsafeBufferUsage/tu-summary-serialization.test)
covering the removed unit test.
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Co-authored-by: Balázs Benics <benicsbalazs@gmail.com>
Co-authored-by: Jan Korous <jkorous@apple.com>
