auto merge of #19357 : michaelwoerister/rust/fix-issue-18791, r=alexcrichton

One negative side-effect of this change is that there might be quite a bit of copying strings out of the codemap, i.e. one copy for every block that gets translated, just for taking a look at the last character of the block. If this turns out to cause a performance problem then `CodeMap::span_to_snippet()` could be changed return `Option<&str>` instead of `Option<String>`.

Fixes #18791

diff --cc src/librustc_trans/trans/base.rs
Simple merge

diff --cc src/librustc_trans/trans/controlflow.rs
Simple merge

diff --cc src/librustc_trans/trans/expr.rs
Simple merge