[cpp.cond] Keywords are not identifiers while preprocessing (#8518)

Author: AlisdairM

source/preprocessor.tex

@@ -553,7 +553,7 @@
 \grammarterm{has-embed-expression}s, and
 \grammarterm{has-attribute-expression}s
 have been performed,
-all remaining identifiers and keywords,
+all remaining identifiers (including those lexically identical to keywords),
 except for
 \tcode{true}
 and