Sema: Add request to compute if an AccessorDecl is transparent

Implicit accessors are sometimes transparent for performance reasons.
Previously this was done in Sema by maybeMarkTransparent(), which would
add a TransparentAttr. Replace this with a request.

lib/Serialization/Serialization.cpp

@@ -3568,6 +3568,7 @@ public:
                                rawAccessorKind,
                                rawAccessLevel,
                                fn->needsNewVTableEntry(),
+                               fn->isTransparent(),
                                dependencies);
 
     writeGenericParams(fn->getGenericParams());