lower bodies' params to thir before the body's value
Two motivations for this:
- Lowering params first means errors from lowering the params are emitted before errors from lowering the body's expression. This comes up in [tests/ui/associated-consts/associated-const-type-parameter-pattern.stderr](https://github.com/rust-lang/rust/compare/master...dianne:rust:thir-lower-params-before-body-expr?expand=1#diff-acac6ea10e991af0da91633e08b2739f9f9ca0c8f826401b6ba829914d0806f2), where both the params and expression encounter errors in translating consts to patterns. This change puts the errors in the order they appear in the source file.
- Guard patterns (#129967) contain expressions, so lowering params containing guard patterns may add more expressions to the THIR. However, there's a check for `-Zunpretty=thir-tree` that the final expression in the THIR corresponds to its value [(link)](c43786c9b7/compiler/rustc_mir_build/src/builder/mod.rs (L453-L455)); lowering params last would break this. As an alternative way to get guard patterns to work, I think the pretty-printer could use the expression returned by `thir_body` and the check could be removed or changed (#141357).
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