[lex.phases] Reorder the first two sentences of phase 7 (#7432)

Currently, the first sentence refers to "tokens" that do not exist until after the second sentence.

Author: AlisdairM

source/lex.tex

@@ -179,10 +179,10 @@
 \item
 Adjacent \grammarterm{string-literal} tokens are concatenated\iref{lex.string}.
 
-\item Whitespace characters separating 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
+\item
+Each preprocessing token is converted into a token\iref{lex.token}.
+Whitespace characters separating tokens are no longer significant.
+The resulting tokens constitute a \defn{translation unit} and
 are syntactically and
 semantically analyzed as a \grammarterm{translation-unit}\iref{basic.link} and
 translated.