Credit goes to dmajor.
Rust gets LLVM 12 - https://github.com/rust-lang/rust/pull/81451.
The PGO format changed between 11 and 12, and builds will have poor optimization if we mix versions in xLTO.
The above PR has landed on Rust nightly 1.52.
Until we move to 1.52 nightly we should have this landed.
In the meantime, for unblocking try builds, we can disable value-PGO in Rust.
Differential Revision: https://phabricator.services.mozilla.com/D113327
