Rollup merge of #153190 - JonathanBrouwer:subdiag_variables, r=jdonszelmann

Don't allow subdiagnostic to use variables from their parent

Tangentially related to rust-lang/rust#151366

This is PR 1/2 for structured diagnostics, will do the unstructured ones next. After the second PR I will be able to remove some code that should compensate for this PR being positive.

Regardless of this PR having a positive diff, I feel that subdiagnostics being able to use variables from their parent is very confusing, so this is for the better,.

r? @jdonszelmann

Author: JonathanBrouwer