[Sema] Fix `ExtVectorElementExpr` tree transform for the `isArrow` case.

Make sure we propagate the value for `IsArrow` to
`RebuildExtVectorElementExpr` in order to be able to handle correctly
vector component accesses where the base value is a pointer.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D131698