[SILGen] Separate the initialization of a wrapped property from a wrapped value

Teach SILGen to emit a separate SIL function to capture the
initialization of the backing storage type for a wrapped property
based on the wrapped value. This eliminates manual code expansion at
every use site.

include/swift/SIL/SILDeclRef.h

@@ -133,6 +133,10 @@ struct SILDeclRef {
     /// routines have an ivar destroyer, which is emitted as
     /// .cxx_destruct.
     IVarDestroyer,
+
+    /// References the wrapped value injection function used to initialize
+    /// the backing storage property from a wrapped value.
+    PropertyWrapperBackingInitializer,
   };
   
   /// The ValueDecl or AbstractClosureExpr represented by this SILDeclRef.