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

[Nikita Popov via llvm-commits]

nikic wrote:

max-rss is measured using `time -f "%M;%e"`, which should produce the same data as `time -l`. Note that this is only for the final linker invocation, so you'd want to run `ninja -v` to get the final `clang` invocation that performs LTO and run that under `time`. (For actual analysis, I would run under `massif`.)

It's possible that there is some platform-specific difference here (libstdc++ vs libc++), but it seems unlikely that that this causes a 10% regression on Linux and no change on MacOS.

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