Don't pass -Werror to ClangImporter's clang when Swift is used from within LLDB.

This works a around search path problems caused by a module existing
in both the build/install directory and the source directory.

<rdar://problem/35714074>

lib/Frontend/CompilerInvocation.cpp

@@ -1234,6 +1234,7 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
     Opts.PCHDisableValidation |= Args.hasArg(OPT_pch_disable_validation);
   }
 
+  Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
   return false;
 }