[lex.phases] Use preprocessing token consistently (#7361)

Prior to converting preprocessing tokens to tokens in phase 7,
all tokens are strictly preprocessing tokens.

Author: AlisdairM

source/lex.tex

@@ -170,14 +170,14 @@
 All preprocessing directives are then deleted.
 
 \item
-For a sequence of two or more adjacent \grammarterm{string-literal} tokens,
+For a sequence of two or more adjacent \grammarterm{string-literal} preprocessing tokens,
 a common \grammarterm{encoding-prefix} is determined
 as specified in \ref{lex.string}.
-Each such \grammarterm{string-literal} token is then considered to have
+Each such \grammarterm{string-literal} preprocessing token is then considered to have
 that common \grammarterm{encoding-prefix}.
 
 \item
-Adjacent \grammarterm{string-literal} tokens are concatenated\iref{lex.string}.
+Adjacent \grammarterm{string-literal} preprocessing tokens are concatenated\iref{lex.string}.
 
 \item
 Each preprocessing token is converted into a token\iref{lex.token}.