bug#40421: 26.2; (And 27.0.90) Inconsistent fontifying in c-mode

[Rolf Ade]

Recipt:

emacs -Q

Insert:

/* 11 */
EXTERN int              XML_GetCurrentByteCount(XML_Parser parser);
/* 12 */
EXTERN enum XML_Status  XML_SetBase(XML_Parser parser, const XML_Char *base);
/* 13 */
EXTERN const XML_Char *  XML_GetBase(XML_Parser parser);
/* 14 */
EXTERN int              XML_GetSpecifiedAttributeCount(XML_Parser parser);
/* 15 */

and M-x c-mode. The font-look seems randomly. Some EXTERN colered, some
not, some function names black, some blue.

The whole (small) file is here
http://tdom.org/index.html/raw/generic/tdomDecls.h?name=cccb9227. With
this syntactically correct file (at least gcc and clang consume it
without complain) there is similiar font-lock flicker around the lines
commented with 12, 13, 14.

I report this (and see it) on my working-place emacs-26.2 but have
verified, that a freshly build emacs-27.0.90 shows the same behaviour.