This is a reland of 7d1f95d6

The reland fixes a performance issue in that we incorrectly marked
every pattern containing a backslash as needing to be escaped,
resulting in a new string allocation instead of reusing the existing
string.

Original change's description:
> [regexp] Correctly escape a backslash-newline sequence
>
> When printing the source string, a backslash-newline sequence ('\\\n',
> '\\\r', '\\\u2028', '\\\u2029') should be formatted as '\n', '\r',
> '\u2028', '\u2029', respectively. Prior to this CL it was formatted as
> a backslash followed by the literal newline character.
>
> Bug: v8:8615
> Change-Id: Iac90195c56ea1707ea8469066b0cc967ea87fc73
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016583
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Auto-Submit: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#65986}

Bug: v8:8615,chromium:1046678
Change-Id: I5d75904f1ea543ec679649668e54749821116442
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2074159
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66476}