bug#48670: 27.2; cc-mode and constexpr if

[John Eivind Helset]

cc-mode has problems with constexpr-if-statements whose true-statement
is not a compound statement.

>From 'emacs -Q' open a buffer in cc-mode and compare the formatting of:

void f(){
  if(a == 4)
    return 4;
  else if(a == 3)
    return 3;
  else
    return 2;
}

void f_constexpr(){
  if constexpr(a == 4)
                return 4;
  else if constexpr(a == 3)
         return 3;
  else
    return 2;
}


In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo 
version 1.17.4)
 of 2021-03-26 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Arch Linux

Recent messages:
Loading /home/jeh/.config/emacs/custom.el (source)...done
Turning on magit-auto-revert-mode...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Package cl is deprecated

Configured using:
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Load-path shadows:
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Memory information:
((conses 16 557252 409165)
 (symbols 48 41880 568)
 (strings 32 164504 45552)
 (string-bytes 1 4903026)
 (vectors 16 61227)
 (vector-slots 8 754212 496078)
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 (buffers 1000 11))

-- 
John Eivind Helset