Linting shouldn't be run on optimized MIR

For example, const-checking needs to run before optimizations, otherwise it might miss some invalid operations that get optimized away. Specifically, it should run on the result of the mir_const query, but currently it uses optimized_mir. This is because the result of the mir_const query is "stolen" instead of cloned, so it no longer exists when clippy runs. clippy tries to work around this by setting mir-opt-level=0, but this doesn't disable every optimization pass.

Until this is fixed, missing_const_for_fn will have false positives and/or ICE (due to some assertions about the state of the MIR at the point const-checking is run).

Start by tracing how clippy obtains MIR for the missing_const_for_fn lint, comparing the mir_const and optimized_mir queries. Investigate why mir_const is stolen before clippy runs and how mir-opt-level=0 affects the optimization passes. Done means linting uses the pre-optimization MIR without false positives or ICEs from const-checking assertions.