librustc_trans: Turn PGO diagnostics into warnings.

They should at least be that, they usually warn about control flow mismatches,
and or the profile being useless, which looks like at least a warning to me.
Signed-off-by: NEmilio Cobos Álvarez <emilio@crisal.io>

src/librustc_trans/back/write.rs

@@ -502,7 +502,7 @@ fn drop(&mut self) {
             let msg = llvm::build_string(|s| {
                 llvm::LLVMRustWriteDiagnosticInfoToString(diagnostic_ref, s)
             }).expect("non-UTF8 PGO diagnostic");
-            diag_handler.note_without_error(&msg);
+            diag_handler.warn(&msg);
         }
         llvm::diagnostic::UnknownDiagnostic(..) => {},
     }