This replaces tabs earlier in the diagnostics emitting process, which allows
various margin calculations to ignore the existence of tabs. It does add a
string copy for the source lines that are emitted.
Properly handle lint spans after MIR inlining
The first commit shows what happens when we apply mir inlining and then cause lints on the inlined MIR.
The second commit fixes that.
r? `@wesleywiser`
