[lex.string] Adopt plain-string-literal grammar to simplify string literals

There are several places in the grammar where we want to refer to a
plain character string literal without a prefix and that is not raw.
Introduce _character-string_ into the grammar to replace a quoted
_s-char-sequence_ in a number of places, and move the `R` of a raw
string literal into the _raw-string_ grammar so that _raw-string_
and _character-string_ are used consistently where they appear.

Author: AlisdairM

source/lex.tex

@@ -613,7 +613,7 @@
 parenthesis is identified. The raw string literal is defined as the shortest sequence
 of characters that matches the raw-string pattern
 \begin{ncbnf}
-\opt{encoding-prefix} \terminal{R} raw-string
+\opt{encoding-prefix} raw-string
 \end{ncbnf}
 
 \item Otherwise, if the next three characters are \tcode{<::} and the subsequent character
@@ -1799,8 +1799,13 @@
 \indextext{literal!string}%
 \begin{bnf}
 \nontermdef{string-literal}\br
-    \opt{encoding-prefix} \terminal{"} \opt{s-char-sequence} \terminal{"}\br
-    \opt{encoding-prefix} \terminal{R} raw-string
+    \opt{encoding-prefix} plain-string-literal\br
+    \opt{encoding-prefix} raw-string
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{plain-string-literal}\br
+    \terminal{"} \opt{s-char-sequence} \terminal{"}
 \end{bnf}
 
 \begin{bnf}
@@ -1823,7 +1828,7 @@
 
 \begin{bnf}
 \nontermdef{raw-string}\br
-    \terminal{"} \opt{d-char-sequence} \terminal{(} \opt{r-char-sequence} \terminal{)} \opt{d-char-sequence} \terminal{"}
+    \terminal{R} \terminal{"} \opt{d-char-sequence} \terminal{(} \opt{r-char-sequence} \terminal{)} \opt{d-char-sequence} \terminal{"}
 \end{bnf}
 
 \begin{bnf}
@@ -2110,12 +2115,10 @@
 
 \begin{bnf}
 \nontermdef{unevaluated-string}\br
-    string-literal
+    plain-string-literal\br
+    raw-string
 \end{bnf}
 
-\pnum
-An \grammarterm{unevaluated-string} shall have no \grammarterm{encoding-prefix}.
-
 \pnum
 Each \grammarterm{universal-character-name} and each \grammarterm{simple-escape-sequence} in an \grammarterm{unevaluated-string} is
 replaced by the member of the translation character set it denotes.

source/preprocessor.tex

@@ -1805,7 +1805,7 @@
 token in the replacement list.
 
 \pnum
-A \defn{character string literal} is a \grammarterm{string-literal} with no prefix.
+A \defn{character string literal} is a \grammarterm{plain-string-literal}.
 If, in the replacement list, a parameter is immediately
 preceded by a
 \tcode{\#}
@@ -2070,12 +2070,6 @@
 \indextext{preprocessing directive!line control}%
 \indextext{\idxcode{\#line}|see{preprocessing directive, line control}}
 
-\pnum
-The \grammarterm{string-literal} of a
-\tcode{\#line}
-directive, if present,
-shall be a character string literal\iref{cpp.stringize}.
-
 \pnum
 The
 \defn{line number}
@@ -2102,11 +2096,11 @@
 \pnum
 A preprocessing directive of the form
 \begin{ncsimplebnf}
-\terminal{\# line} digit-sequence \terminal{"} \opt{string-literal} \terminal{"} new-line
+\terminal{\# line} digit-sequence plain-string-literal new-line
 \end{ncsimplebnf}
 sets the presumed line number similarly and changes the
 presumed name of the source file to be the contents
-of the character string literal.
+of the \grammarterm{plain-string-literal}.
 
 \pnum
 A preprocessing directive of the form