[lex.phases] Move cross-reference to the first use of the referenced term (#7393)

Author: lprv

source/lex.tex

@@ -90,7 +90,7 @@
 does not constitute an endorsement by ISO or IEC of this product.
 \end{footnote}
 scalar values.
-A sequence of translation character set elements is then formed
+A sequence of translation character set elements\iref{lex.charset} is then formed
 by mapping each Unicode scalar value
 to the corresponding translation character set element.
 In the resulting sequence,
@@ -103,7 +103,7 @@
 For any other kind of input file supported by the implementation,
 characters are mapped, in an
 \impldef{mapping physical source file characters to translation character set} manner,
-to a sequence of translation character set elements\iref{lex.charset},
+to a sequence of translation character set elements,
 representing end-of-line indicators as new-line characters.
 
 \item