Don't revisit the subexpressions of PseudoObjectExpr when building a

ParentMap

The assertion that is removed in this patch was failing when ObjC dot
notation expressions appear in both sides of an assignment (see the test
case in arc-repeated-weak.mm). Visit the PseudoObjectExpr once when the
syntactic expression is visited and return without visiting the
subexpressions when it's visited again when the semantic expressions are
visited.

Differential Revision: https://reviews.llvm.org/D139171