[lex] Provide unicode name for all control characters

This commit does not touch the new-line character as paper
P2348.  It resricts itself to consistent use of the unicode
character name for space, horizontal tab, and vertical tab.
Compared to PR cplusplus#7359 it deliberately does not touch the
grammar that would necessitate a review by core review.
The intent is to rebase that PR if this one lands.

Author: AlisdairM

source/lex.tex

@@ -140,9 +140,9 @@
 would arise from a source file ending with an unclosed \tcode{/*}
 comment.
 \end{footnote}
-Each comment\iref{lex.comment} is replaced by one space character. New-line characters are
+Each comment\iref{lex.comment} is replaced by one \unicode{0020}{space} character. New-line characters are
 retained. Whether each nonempty sequence of whitespace characters other
-than new-line is retained or replaced by one space character is
+than new-line is retained or replaced by one \unicode{0020}{space} character is
 unspecified.
 As characters from the source file are consumed
 to form the next preprocessing token
@@ -882,7 +882,8 @@
 \end{footnote}
 operators, and other separators.
 \indextext{whitespace}%
-Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments
+Comments and the characters \unicode{0020}{space}, \unicode{0009}{character tabulation},
+\unicode{0009}{line tabulation}, \unicode{000c}{form feed}, and new-line
 (collectively, ``whitespace''), as described below, are ignored except
 as they serve to separate tokens.
 \begin{note}