[lex.phases] Reorder the first two sentences of phase 7

As suggested by Jens Maurer.  The first sentence refers to "tokens" that
do not exist until after the second sentence.  Rather than change normaitve
meaning by referring to the extant "preprocessing tokens" is seems best
to preserve the clear intent, and simply reverse their order.

Author: AlisdairM

source/lex.tex

@@ -178,10 +178,10 @@
 \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 and translated.
 \begin{note}