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/ModuleFormat.h

@@ -586,6 +586,7 @@ namespace decls_block {
     BCFixed<1>,   // IBOutlet?
     BCFixed<1>,   // optional?
     BCFixed<1>,   // static?
+    BCFixed<1>,   // isLet?
     TypeIDField,  // type
     TypeIDField,  // interface type
     DeclIDField,  // getter