Remove BorrowckErrors::tainted_by_errors.

`BorrowckErrors` stores a mix of error and non-error diags in
`buffered`. As a result, it downgrades `DiagnosticBuilder`s to
`Diagnostic`s, losing the emission guarantees, and so has to use a
`tainted_by_errors` field to record whether an error has occurred.

This commit splits `buffered` into `buffered_errors` and
`buffered_non_errors`, keeping them as `DiagnosticBuilder`s and
preserving the emission guarantees.

This also requires fixing a bunch of incorrect lifetimes on
`DiagnosticBuilder` use points.

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -288,7 +288,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         &mut self,
         place: Place<'tcx>,
         span: Span,
-    ) -> DiagnosticBuilder<'a> {
+    ) -> DiagnosticBuilder<'tcx> {
         let description = if place.projection.len() == 1 {
             format!("static item {}", self.describe_any_place(place.as_ref()))
         } else {
@@ -310,7 +310,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         deref_target_place: Place<'tcx>,
         span: Span,
         use_spans: Option<UseSpans<'tcx>>,
-    ) -> DiagnosticBuilder<'a> {
+    ) -> DiagnosticBuilder<'tcx> {
         // Inspect the type of the content behind the
         // borrow to provide feedback about why this
         // was a move rather than a copy.