[Bash-autocompletion] Add support for -W<warning> and -Wno<warning>

Summary:
`-W[tab]` will autocomplete warnings defined in this link:
https://clang.llvm.org/docs/DiagnosticsReference.html#wweak-vtables

Differential Revision: https://reviews.llvm.org/D35447

llvm-svn: 308139
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 4247801..1d35d6e 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -1275,6 +1275,13 @@
       // we were requested to print out all option names that start with "-foo".
       // For example, "--autocomplete=-fsyn" is expanded to "-fsyntax-only".
       SuggestedCompletions = Opts->findByPrefix(PassedFlags, DisableFlags);
+
+      // We have to query the -W flags manually as they're not in the OptTable.
+      // TODO: Find a good way to add them to OptTable instead and them remove
+      // this code.
+      for (StringRef S : DiagnosticIDs::getDiagnosticFlags())
+        if (S.startswith(PassedFlags))
+          SuggestedCompletions.push_back(S);
     } else {
       // If the flag is in the form of "--autocomplete=foo,bar", we were
       // requested to print out all option values for "-foo" that start with