Downgrade unimplemented mangling diagnostic from error to note.
This codepath is exercised by MozsearchIndexer.cpp (the searchfox
indexer) when indexing on Windows. We can do without having the
unimplemented bits for now as long the compiler doesn't fail the
build. See also https://bugs.llvm.org/show_bug.cgi?id=39294

diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 8b1419074df5..4436cd118f87 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -3847,10 +3847,11 @@ recurse:
     if (!NullOut) {
       // As bad as this diagnostic is, it's better than crashing.
       DiagnosticsEngine &Diags = Context.getDiags();
-      unsigned DiagID = Diags.getCustomDiagID(DiagnosticsEngine::Error,
+      unsigned DiagID = Diags.getCustomDiagID(DiagnosticsEngine::Remark,
                                        "cannot yet mangle expression type %0");
       Diags.Report(E->getExprLoc(), DiagID)
         << E->getStmtClassName() << E->getSourceRange();
+      Out << "MOZ_WE_HACKED_AROUND_BUG_1418415";
     }
     break;
   }