This escapes LineTerminator characters in a regexp pattern when
creating the string that will be stored in the [[OriginalSource]] slot.

As an example, the source property for all following objects will equal
"\n" (a '\' character followed by 'n'):

  /\n/
  new RegExp("\n")
  new RegExp("\\n")

Bug: v8:1982, chromium:855009
Change-Id: I3b539497a0697e3d51ec969cae49308b0b312a19
Reviewed-on: https://chromium-review.googlesource.com/c/1384316
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58387}