[lex.phases] Move dropping whitespace to end of phase 4 (#8117)

Move the dropping of whitespace to the end of phase 4, after preprocessing directives are deleted.

Author: AlisdairM

source/lex.tex

@@ -157,6 +157,7 @@
 A \tcode{\#include} preprocessing directive\iref{cpp.include} causes the named header or
 source file to be processed from phase 1 through phase 4, recursively.
 All preprocessing directives are then deleted.
+Whitespace characters separating preprocessing tokens are no longer significant.
 
 \item
 For a sequence of two or more adjacent \grammarterm{string-literal} preprocessing tokens,
@@ -171,7 +172,6 @@
 
 \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