UPDATE - move SessionDiagnostic from rustc_session to rustc_errors

compiler/rustc_errors/src/lib.rs

@@ -60,6 +60,7 @@ mod snippet;
 mod styled_buffer;
 pub mod translation;
 
+pub use diagnostic_builder::SessionDiagnostic;
 pub use snippet::Style;
 
 pub type PResult<'a, T> = Result<T, DiagnosticBuilder<'a, ErrorGuaranteed>>;