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}