Ensure proper mode of *Compile-Log* buffer (bug#67920)

Reported by OGAWA Hirofumi.

* lisp/emacs-lisp/bytecomp.el (displaying-byte-compile-warnings):
Move most of the innards to...
(bytecomp--displaying-warnings): ...this new function, for ease
of maintenance.
* lisp/emacs-lisp/bytecomp.el (byte-compile-file):
Wrap early warning about missing lexbind declaration in
`displaying-byte-compile-warnings` so that it doesn't cause the
creation of a compile-log buffer with the wrong mode.
2 jobs for master in 97 minutes and 35 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Build Images
failed #78771
build-image-inotify

01:37:35

 
  Normal
skipped #78772
test-lisp-emacs-lisp-inotify
 
Name Stage Failure
failed
build-image-inotify Build Images There has been a timeout failure or the job got stuck. Check your timeout limits or try again
#11 96.44 checking POSIX termios... yes
#11 96.64 checking size of speed_t... 4
#11 96.83 checking for usable FIONREAD... yes
#11 96.93 checking for usable SIGIO... yes
#11 97.06 checking for struct alignment... yes
#11 97.14 checking for C/C++ restrict keyword... __restrict__
#11 97.25 checking for typeof syntax and keyword spelling... typeof
#11 97.32 checking for statement expressions... yes
#11 97.42 checking whether malloc (0) returns nonnull...
ERROR: Job failed (system failure): aborted: terminated