[LLDB] Fix NSIndexPathSyntheticFrontEnd::Impl::Clear() to only clear the active union member

NSIndexPathSyntheticFrontEnd::Impl::Clear() currently calls Clear() on both
unions members regardless of which one is active. I modified it to only call
Clear() on the active member.

Differential Revision: https://reviews.llvm.org/D122753

lldb/source/Plugins/Language/ObjC/NSIndexPath.cpp

@@ -282,9 +282,17 @@ protected:
     };
 
     void Clear() {
+      switch (m_mode) {
+      case Mode::Inlined:
+        m_inlined.Clear();
+        break;
+      case Mode::Outsourced:
+        m_outsourced.Clear();
+        break;
+      case Mode::Invalid:
+        break;
+      }
       m_mode = Mode::Invalid;
-      m_inlined.Clear();
-      m_outsourced.Clear();
     }
 
     Impl() {}