TextDiagnosticPrinter: use the mapped level for remark flag computation

Custom diagnostics don't have a builtin class so this wouldn't have worked.
Reduces surface area of remark-related changes.

No test coverage.

llvm-svn: 211462
diff --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index d6df655..1d34abf 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -82,7 +82,7 @@
     StringRef Opt = DiagnosticIDs::getWarningOptionForDiag(Info.getID());
     if (!Opt.empty()) {
       OS << (Started ? "," : " [")
-         << (DiagnosticIDs::isRemark(Info.getID()) ? "-R" : "-W") << Opt;
+         << (Level == DiagnosticsEngine::Remark ? "-R" : "-W") << Opt;
       StringRef OptValue = Info.getDiags()->getFlagNameValue();
       if (!OptValue.empty())
         OS << "=" << OptValue;