Always use lexical-binding in lisp-interaction-mode (bug#38835)

* lisp/progmodes/elisp-mode.el (lisp-interaction-mode):
Set lexical-binding.
* lisp/startup.el (command-line, startup--get-buffer-create-scratch):
Don't set lexical-binding here.
* doc/lispref/variables.texi:
* etc/NEWS:
Make it clear that lisp-interaction-mode uses lexical-binding.

doc/lispref/variables.texi

@@ -1187,6 +1187,9 @@ When evaluating Emacs Lisp code directly using an @code{eval} call,
 lexical binding is enabled if the @var{lexical} argument to
 @code{eval} is non-@code{nil}.  @xref{Eval}.
 
+Lexical binding is also enabled in Lisp Interaction and IELM
+mode, used in the @file{*scratch*} and @file{*ielm*} buffers.
+
 @cindex special variables
   Even when lexical binding is enabled, certain variables will
 continue to be dynamically bound.  These are called @dfn{special

etc/NEWS

@@ -338,7 +338,8 @@ answers, but also function keys like 'F1', character events such as
 
 ** Lexical binding is now used when evaluating interactive Elisp forms.
 More specifically, lexical-binding is now used for 'M-:', '--eval', as
-well as in the "*scratch*" and "*ielm*" buffers.
+well as in lisp-interaction-mode and ielm-mode, used in the "*scratch*"
+and "*ielm*" buffers.
 
 ---
 ** The new user option 'tooltip-resize-echo-area' avoids truncating

lisp/progmodes/elisp-mode.el

@@ -933,7 +933,8 @@ Paragraphs are separated only by blank lines.
 Semicolons start comments.
 
 \\{lisp-interaction-mode-map}"
-  :abbrev-table nil)
+  :abbrev-table nil
+  (setq-local lexical-binding t))
 
 ;;; Emacs Lisp Byte-Code mode
 

lisp/startup.el

@@ -1435,8 +1435,7 @@ please check its value")
   (if (get-buffer "*scratch*")
       (with-current-buffer "*scratch*"
 	(if (eq major-mode 'fundamental-mode)
-	    (funcall initial-major-mode))
-        (setq-local lexical-binding t)))
+	    (funcall initial-major-mode))))
 
   ;; Load library for our terminal type.
   ;; User init file can set term-file-prefix to nil to prevent this.
@@ -2317,7 +2316,6 @@ A fancy display is used on graphic displays, normal otherwise."
   (or (get-buffer "*scratch*")
       (with-current-buffer (get-buffer-create "*scratch*")
         (set-buffer-major-mode (current-buffer))
-        (setq-local lexical-binding t)
         (current-buffer))))
 
 (defun command-line-1 (args-left)