Add a field reflection function that constructs keypaths. (#34815)

While the existing _forEachField in ReflectionMirror.swift
already gives the offsets and types for each field, this isn't
enough information to construct a keypath for that field in
order to modify it.

For reference, this should be sufficent to implement the features
described here: (https://forums.swift.org/t/storedpropertyiterable/19218/62)
purely at runtime without any derived conformances for many types.

Note: Since there isn't enough reflection information for
`.mutatingGetSet` fields, this means that we're not able to support
reflecting certain types of fields (functions, nonfinal class fields,
etc). Whether this is an error or not is controlled by the `.ignoreUnknown`
option.

include/swift/Reflection/Records.h

@@ -92,6 +92,10 @@ public:
   bool isIndirectCase() const {
     return Flags.isIndirectCase();
   }
+
+  bool isVar() const {
+    return Flags.isVar();
+  }
 };
 
 struct FieldRecordIterator {