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/interpreter.h

@@ -75,9 +75,6 @@ class Interpreter {
   auto EvalPrim(Operator op, const std::vector<Nonnull<const Value*>>& args,
                 SourceLocation source_loc) -> Nonnull<const Value*>;
 
-  void PatternAssignment(Nonnull<const Value*> pat, Nonnull<const Value*> val,
-                         SourceLocation source_loc);
-
   // Returns the result of converting `value` to type `destination_type`.
   auto Convert(Nonnull<const Value*> value,
                Nonnull<const Value*> destination_type) const