Clarify and simplify handling of lvalues (#956)

- Rename `PointerValue` to `LValue` to reflect how it's actually used. We can introduce a `PointerValue` type when we add support for actual pointer values.
- Remove support for pattern assignment. It's unclear if Carbon will support this, and even if we do, it raises questions that should first be addressed in a language proposal, like "is the left-hand side of `(x, y) = (1, 2)` an lvalue, or a pattern, or both, or something else entirely?"

executable_semantics/interpreter/action.h

@@ -132,8 +132,7 @@ class Action {
 };
 
 // An Action which implements evaluation of an Expression to produce an
-// lvalue. The result be expressed as a PointerValue which points to the
-// Expression's value.
+// LValue.
 class LValAction : public Action {
  public:
   explicit LValAction(Nonnull<const Expression*> expression)