Build a better MIR body when errors are encountered
Doesn't really have much of an effect on its own, but it does lead to a less confusing phony MIR body being generated when an error is detected during THIR/MIR/match building. This was quite confusing when I hacked `-Zunpretty=mir` to emit `mir_built` rather than `instance_mir`.
This coincidentually also fixes#117413, but not as generally as #117416.
cc `@Nadrieril`
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