[LLDB] Add LLDB accessor for ModuleInterfaceLoaderOptions

LLDB needs to control whether implicit Swift modules are on or off within a
single ASTContext:

- when importing the context of a Swift module in an EBM
  project only explicit modules should be allowed

- when evaluating arbitrary expressions (which may include `import` directives)
  implicit modules should be enabled

rdar://166494895

Author: adrian-prantl

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -558,6 +558,9 @@ class ModuleInterfaceLoader : public SerializedModuleLoaderBase {
                                 IgnoreSwiftSourceInfoFile));
   }
 
+  /// Accessor used by LLDB.
+  ModuleInterfaceLoaderOptions &getOptions();
+
   /// Append visible module names to \p names. Note that names are possibly
   /// duplicated, and not guaranteed to be ordered in any way.
   void collectVisibleTopLevelModuleNames(

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1304,6 +1304,10 @@ ModuleInterfaceLoaderOptions::ModuleInterfaceLoaderOptions(
   }
 }
 
+ModuleInterfaceLoaderOptions &ModuleInterfaceLoader::getOptions() {
+  return InterfaceChecker.Opts;
+}
+
 bool ModuleInterfaceLoader::isCached(StringRef DepPath) {
   return InterfaceChecker.isCached(DepPath);
 }