Stop using DiagnosticBuilder::buffer in BorrowckErrors.

But we can't easily switch from `Vec<Diagnostic>` to
`Vec<DiagnosticBuilder<G>>` because there's a mix of errors and warnings
which result in different `G` types. So we must make
`DiagnosticBuilder::into_diagnostic` public, but that's ok, and it will
get more use in subsequent commits.

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -260,7 +260,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     }
 
     /// Converts the builder to a `Diagnostic` for later emission.
-    fn into_diagnostic(mut self) -> Diagnostic {
+    pub fn into_diagnostic(mut self) -> Diagnostic {
         self.take_diag()
     }