defrost `RUST_MIN_STACK=ice rustc hello.rs`
I didn't think too hard about testing my previous PR rust-lang/rust#122847 which makes our stack overflow handler assist people in discovering the `RUST_MIN_STACK` variable (which apparently is surprisingly useful for Really Big codebases). After it was merged, some useful comments left in a drive-by review led me to discover I had added an ICE. This reworks the code a bit to explain the rationale, remove the ICE that I introduced, and properly test one of the diagnostics.
ecbd110c7e