Introduce a new bit in VarDecl, "isLet". Teach Sema that 'isLet' properties

are not settable (like get-only ones).  Set the 'isLet' bit in various 
places, but not the particularly interesting or useful places yet.



Swift SVN r11121

lib/Serialization/Serialization.cpp

@@ -1202,6 +1202,7 @@ void Serializer::writeDecl(const Decl *D) {
                           var->getAttrs().isIBOutlet(),
                           var->getAttrs().isOptional(),
                           var->isStatic(),
+                          var->isLet(),
                           addTypeRef(type),
                           addTypeRef(var->getInterfaceType()),
                           addDeclRef(var->getGetter()),