Don't keep warning about unescaped literals (Bug#36068)

* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Restore lost
let-binding of lread--unescaped-character-literals, so that unescaped
literals warning will only apply to the form just read.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--unescaped-char-literals): Expand test to check that
we don't keep warning about old unescaped literals.
1 job for master in 4 seconds (queued for 1 second)
Status Job ID Name Coverage
failed #2102


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)