Auto merge of #95968 - davidtwco:translation-lazy-fallback, r=oli-obk

errors: lazily load fallback fluent bundle

Addresses (hopefully) https://github.com/rust-lang/rust/pull/95667#issuecomment-1094794087.

Loading the fallback bundle in compilation sessions that won't go on to emit any errors unnecessarily degrades compile time performance, so lazily create the Fluent bundle when it is first required.

r? `@ghost` (just for perf initially)