Get all (but one) of debuginfo tests to pass with MIR codegen.
I didn't get much feedback in #31005 so I went ahead and implemented something simple.
Closes#31005, as MIR debuginfo should work now for most usecases.
The `no-debug-attribute` test no longer assumes variables are in scope of `return`.
We might also want to revisit that in #32949, but the test is more reliable now either way.
In order to get one last function in the `associated-type` test pass, this PR also fixes#32790.
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