[llvm] [InstrRef][nfc] Remove usage of unique_ptrs of arrays (PR #74203)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

This seems fine to me and is probably clearer than the array-allocation situation. I've added a question about a potentially functional change / code-smell inline.

(I wrote the unique_ptr stuff in D118774 after having been bitten by asan too many times, in a bid to escape further manual memory allocation).

https://github.com/llvm/llvm-project/pull/74203