Generalize storage implementations to support generalized accessors.

The storage kind has been replaced with three separate "impl kinds",
one for each of the basic access kinds (read, write, and read/write).
This makes it far easier to mix-and-match implementations of different
accessors, as well as subtleties like implementing both a setter
and an independent read/write operation.

AccessStrategy has become a bit more explicit about how exactly the
access should be implemented.  For example, the accessor-based kinds
now carry the exact accessor intended to be used.  Also, I've shifted
responsibilities slightly between AccessStrategy and AccessSemantics
so that AccessSemantics::Ordinary can be used except in the sorts of
semantic-bypasses that accessor synthesis wants.  This requires
knowing the correct DC of the access when computing the access strategy;
the upshot is that SILGenFunction now needs a DC.

Accessor synthesis has been reworked so that only the declarations are
built immediately; body synthesis can be safely delayed out of the main
decl-checking path.  This caused a large number of ramifications,
especially for lazy properties, and greatly inflated the size of this
patch.  That is... really regrettable.  The impetus for changing this
was necessity: I needed to rework accessor synthesis to end its reliance
on distinctions like Stored vs. StoredWithTrivialAccessors, and those
fixes were exposing serious re-entrancy problems, and fixing that... well.
Breaking the fixes apart at this point would be a serious endeavor.

Author: rjmccall

include/swift/AST/ASTWalker.h

@@ -201,6 +201,16 @@ class ASTWalker {
   /// params in AbstractFunctionDecl and NominalTypeDecl.
   virtual bool shouldWalkIntoGenericParams() { return false; }
 
+  /// This method configures whether the walker should walk into the
+  /// initializers of lazy variables.  These initializers are semantically
+  /// different from other initializers in their context and so sometimes
+  /// should not be visited.
+  ///
+  /// Note that visiting the body of the lazy getter will find a
+  /// LazyInitializerExpr with the initializer as its sub-expression.
+  /// However, ASTWalker does not walk into LazyInitializerExprs on its own.
+  virtual bool shouldWalkIntoLazyInitializers() { return true; }
+
   /// walkToParameterListPre - This method is called when first visiting a
   /// ParameterList, before walking into its parameters.  If it returns false,
   /// the subtree is skipped.