bug#37137: Setting font-lock-case-fold-search to t causes hangs on certain types in c-mode

[Zachary Napier <zachary.napier.44@gmail.com>]

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I have in my .emacs file only these two lines:

    (custom-set-variables
     '(font-lock-keywords-case-fold-search t))

Then if I visit an empty or nonexistent C file and just type "LONG"
(in all caps or with at least one letter being capital), Emacs will
hang after typing the final G but before the G appears on
screen. Typing C-g a couple of times gets Emacs unstuck until I
continue to type. The same behavior also happens if I type "SHORT".

This happens in both Emacs 26.1 in text mode on a Linux machine and in
Emacs 25.3 in GUI mode on Windows.

After messing with GDB a bit, I am pretty sure the hang is at the
while loop in c-forward-type at cc-engine.el:7657 and its getting
stuck because looking-at matches LONG because its case insensitive,
but the c-forward-keyword-clause doesn't move forward because the
first thing it does is try to find "LONG" in c-keywords-obarray via
`intern-soft` and immediately returns because it doesn't find it
there. Here is the while loop:

    (while (progn
             (setq safe-pos (point))
             (looking-at c-opt-type-component-key))
      (when (and c-record-type-identifiers
                 (looking-at c-primitive-type-key))
        (c-record-type-id (cons (match-beginning 1)
                                (match-end 1))))
      (c-forward-keyword-clause 1))

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