[-Wunsafe-buffer-usage] Improving insertion of the [[clang::unsafe_buffer_usage]] attribute

For a fix-it that inserts the `[[clang::unsafe_buffer_usage]]`
attribute, it will lookup existing macros defined for the attribute
and use the (last defined such) macro directly. Fix-its will use raw
`[[clang::unsafe_buffer_usage]]` if no such macro is defined.

The implementation mimics how a similar machine for the
`[[fallthrough]]` attribute was implemented.

Reviewed by: NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D150338