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.

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -1087,8 +1087,8 @@ static ValueDecl *deriveHashable_hashValue(DerivedConformance &derived) {
   hashValueDecl->setImplicit();
   hashValueDecl->setInterfaceType(intType);
   hashValueDecl->setValidationStarted();
-  hashValueDecl->setAccessors(VarDecl::Computed, SourceLoc(), {getterDecl},
-                              SourceLoc());
+  hashValueDecl->setAccessors(StorageImplInfo::getImmutableComputed(),
+                              SourceLoc(), {getterDecl}, SourceLoc());
   hashValueDecl->copyFormalAccessFrom(derived.Nominal,
                                       /*sourceIsParentContext*/ true);