[clang][ssaf] Add UnsafeBufferUsage summary data structures (#181067)
An UnsafeBufferUsage summary is the ssaf representation of the set of
unsafe buffer pointers found by `-Wunsafe-buffer-usage` in a translation
unit.
rdar://170176278
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Co-authored-by: Balázs Benics <benicsbalazs@gmail.com>