[lex.phases] Use preprocessing token consistently

Prior to converting preprocessing tokens to tokens in phase 7,
all tokens are strictly preprocessing tokens.  Add the missing
qualification is appropriate through the phases of translation
up to that point of conversion.

source/lex.tex

@@ -169,16 +169,16 @@
 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 Whitespace characters separating tokens are no longer
+\item Whitespace characters separating preprocessing tokens are no longer
 significant. Each preprocessing token is converted into a
 token\iref{lex.token}. The resulting tokens
 constitute a \defn{translation unit} and