Prior to this CL we still implemented a HasProperty-GetProperty
sequence when accessing named captures in GetSubstitution. This was
briefly part of the spec (we also threw an exception when the property
was not present), but since late 2017 the GetProperty call has been
unconditional.

See https://tc39.es/ecma262/#sec-getsubstitution.

Bug: v8:10513
Change-Id: Id82c06958b0b0feffc6eede580b99ab8676a0dae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2195821
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67733}