[lex] Complete the use of unicode code points

Resolves #5502

Completes the task of applying Unicode code point markup to
refer to single characters in normative text.

Author: AlisdairM

source/lex.tex

@@ -99,12 +99,12 @@
 \indextext{line splicing}%
 If the first translation character is \unicode{feff}{byte order mark},
 it is deleted.
-Each sequence comprising a backslash character (\textbackslash)
+Each sequence comprising a \unicode{005c}{reverse solidus} character (\textbackslash)
 immediately followed by
 zero or more whitespace characters other than new-line followed by
 a new-line character is deleted, splicing
 physical source lines to form \defnx{logical source lines}{source line!logical}. Only the last
-backslash on any physical source line shall be eligible for being part
+ \unicode{005c}{reverse solidus} on any physical source line shall be eligible for being part
 of such a splice.
 \begin{note}
 Line splicing can form
@@ -577,7 +577,8 @@
 circumstances during translation phase 4, whitespace (or the absence
 thereof) serves as more than preprocessing token separation. Whitespace
 can appear within a preprocessing token only as part of a header name or
-between the quotation characters in a character literal or
+between the \unicode{0027}{apostrophe} characters in a character literal
+or between the \unicode{0022}{quotation mark} characters in a
 string literal.
 
 \pnum
@@ -728,13 +729,14 @@
 \end{note}
 
 \pnum
-The appearance of either of the characters \tcode{'} or \tcode{\textbackslash} or of
+The appearance of either of the characters \unicode{0027}{apostrophe} or
+\unicode{005c}{reverse solidus} or of
 either of the character sequences \tcode{/*} or \tcode{//} in a
 \grammarterm{q-char-sequence} or an \grammarterm{h-char-sequence}
 is conditionally-supported with \impldef{meaning of \tcode{'}, \tcode{\textbackslash},
 \tcode{/*}, or \tcode{//} in a \grammarterm{q-char-sequence} or an
 \grammarterm{h-char-sequence}} semantics, as is the appearance of the character
-\tcode{"} in an \grammarterm{h-char-sequence}.
+\unicode{0022}{quotation mark} in an \grammarterm{h-char-sequence}.
 \begin{note}
 Thus, a sequence of characters
 that resembles an escape sequence can result in an error, be interpreted as the