TBDGen: teach the compiler to take a json file indicating previous install names

Using the new linker directives $ld$previous requires the compiler to know the previous
install names for the symbols marked as removed. This patch teaches the compiler
to take a path to a Json file specifying the map between module names and previous
install names. Also, these install names can be platform-specific.

Progress towards: rdar://58281536

lib/Frontend/CompilerInvocation.cpp

@@ -1018,6 +1018,9 @@ static bool ParseTBDGenArgs(TBDGenOptions &Opts, ArgList &Args,
   if (const Arg *A = Args.getLastArg(OPT_tbd_current_version)) {
     Opts.CurrentVersion = A->getValue();
   }
+  if (const Arg *A = Args.getLastArg(OPT_previous_module_installname_map_file)) {
+    Opts.ModuleInstallNameMapPath = A->getValue();
+  }
   return false;
 }