Add an option to disable ClangImporter imports form source.

This is primarily meant to used for testing LLDB's DWARFImporterDelegate,
however, this could become the default option for LLDB once
DWARFImporterDelegate is sufficiently mature.

<rdar://problem/57880844>

lib/Frontend/CompilerInvocation.cpp

@@ -638,7 +638,12 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
 
   if (Args.hasArg(OPT_warnings_as_errors))
     Opts.ExtraArgs.push_back("-Werror");
+
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
+
+  Opts.DisableSourceImport |=
+      Args.hasArg(OPT_disable_clangimporter_source_import);
+
   return false;
 }