[SE-0258] Promote projection variables ($foo) to the original property access

When the outermost property wrapper associated with a property has a
`wrapperValue`, create the projection property (with the `$` prefix)
at the same access level as the original property. This puts the
wrapped-value interface and the projection interface at the same level.

The newly-introduced @_projectionValueProperty attribute is implicitly
created to establish the link between the original property and the
projection value within module interfaces, where both properties will
be explicitly written out.

include/swift/Serialization/ModuleFormat.h

@@ -52,7 +52,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 497; // remove FuncDecl::hasDynamicSelf()
+const uint16_t SWIFTMODULE_VERSION_MINOR = 498; // projection value attr
 
 using DeclIDField = BCFixed<31>;
 
@@ -1576,6 +1576,11 @@ namespace decls_block {
   using ClangImporterSynthesizedTypeDeclAttrLayout
     = BCRecordLayout<ClangImporterSynthesizedType_DECL_ATTR>;
   using PrivateImportDeclAttrLayout = BCRecordLayout<PrivateImport_DECL_ATTR>;
+  using ProjectionValuePropertyDeclAttrLayout = BCRecordLayout<
+      ProjectionValueProperty_DECL_ATTR,
+      BCFixed<1>,        // isImplicit
+      IdentifierIDField  // name
+  >;
 
   using InlineDeclAttrLayout = BCRecordLayout<
     Inline_DECL_ATTR,