Suppress "erroneous constant used" for constants tainted by errors

When constant evaluation fails because its MIR is tainted by errors,
suppress note indicating that erroneous constant was used, since those
errors have to be fixed regardless of the constant being used or not.

Author: tmiasko

src/diagnostics.rs

@@ -289,7 +289,7 @@ pub fn report_error<'tcx, 'mir>(
                     (None, format!("see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information")),
                 ],
             InvalidProgram(
-                InvalidProgramInfo::AlreadyReported(rustc_errors::ErrorGuaranteed { .. })
+                InvalidProgramInfo::AlreadyReported(_)
             ) => {
                 // This got already reported. No point in reporting it again.
                 return None;

src/lib.rs

@@ -47,7 +47,6 @@ extern crate rustc_ast;
 extern crate rustc_middle;
 extern crate rustc_const_eval;
 extern crate rustc_data_structures;
-extern crate rustc_errors;
 extern crate rustc_hir;
 extern crate rustc_index;
 extern crate rustc_session;