[Frontend] Add a new -emit-interface-path option

...but don't hook it up to anything yet.

This is the very very start of the module stability / textual
interfaces feature described at

  https://forums.swift.org/t/plan-for-module-stability/14551/

For now I've just made it a frontend option (not a driver option),
which is good enough for testing.

lib/Frontend/FrontendInputsAndOutputs.cpp

@@ -418,6 +418,12 @@ bool FrontendInputsAndOutputs::hasModuleDocOutputPath() const {
         return outs.ModuleDocOutputPath;
       });
 }
+bool FrontendInputsAndOutputs::hasModuleInterfaceOutputPath() const {
+  return hasSupplementaryOutputPath(
+      [](const SupplementaryOutputPaths &outs) -> const std::string & {
+        return outs.ModuleInterfaceOutputPath;
+      });
+}
 bool FrontendInputsAndOutputs::hasTBDPath() const {
   return hasSupplementaryOutputPath(
       [](const SupplementaryOutputPaths &outs) -> const std::string & {