[cpp.line] Address string-literal vs s-char-sequence

The text of this specification places a restriction on a string-literal
that is not present in the grammar, although the grammar is an exact
match for a string-literal with that restriction.

The restricted term is also used when specifying the effects of the
line directive with a string, so prefer to move string-literal into
the grammar rather than strike that paragraph and adjust the effects.

Added a cross-reference to where character string literal is defined
as it may not be obvious to all readers that this is a term of power,
even though it is defined as a term and has an index entry.

Author: AlisdairM

source/preprocessor.tex

@@ -2074,7 +2074,7 @@
 The \grammarterm{string-literal} of a
 \tcode{\#line}
 directive, if present,
-shall be a character string literal.
+shall be a character string literal\iref{cpp.stringize}.
 
 \pnum
 The
@@ -2102,7 +2102,7 @@
 \pnum
 A preprocessing directive of the form
 \begin{ncsimplebnf}
-\terminal{\# line} digit-sequence \terminal{"} \opt{s-char-sequence} \terminal{"} new-line
+\terminal{\# line} digit-sequence \terminal{"} \opt{string-literal} \terminal{"} new-line
 \end{ncsimplebnf}
 sets the presumed line number similarly and changes the
 presumed name of the source file to be the contents