Simplify warning flag value handling from r206826

Also give the field it a more appropriate name and improve the docs.

llvm-svn: 212584
diff --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index 1d34abf..6271509 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -83,7 +83,7 @@
     if (!Opt.empty()) {
       OS << (Started ? "," : " [")
          << (Level == DiagnosticsEngine::Remark ? "-R" : "-W") << Opt;
-      StringRef OptValue = Info.getDiags()->getFlagNameValue();
+      StringRef OptValue = Info.getDiags()->getFlagValue();
       if (!OptValue.empty())
         OS << "=" << OptValue;
       Started = true;