coverage: Build the expansion tree around `SyntaxContext`
The expansion tree was previously built around `ExpnId`, which was subtly incorrect in cases where multiple distinct syntax contexts share the same `outer_expn` ID. This can occur in the presence of nested macro expansions.
This change turns out to be fairly straightforward, because in all cases we were obtaining `ExpnId` from `span.ctxt().outer_expn()` anyway, so we can just remove the call to `.outer_expn()` and use the context instead.
As a result of this change, `context-mismatch-issue-147339.rs` no longer triggers a context mismatch in `extract_refined_covspans`, so this PR is a more principled fix for https://github.com/rust-lang/rust/issues/147339.
Jonathan Brouwer
fdf56d2cf4