[cpp.pre] define, index, and consistently use the term 'logical source line'

Author: AlisdairM

source/lex.tex

@@ -114,7 +114,7 @@
 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 logical source lines. Only the last
+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
 of such a splice.
 \begin{note}

source/preprocessor.tex

@@ -150,7 +150,7 @@
 
 \item
 an \keyword{import} preprocessing token
-immediately followed on the same logical line by a
+immediately followed on the same logical source line by a
 \grammarterm{header-name},
 \tcode{<},
 \grammarterm{identifier},
@@ -160,15 +160,15 @@
 
 \item
 a \keyword{module} preprocessing token
-immediately followed on the same logical line by an
+immediately followed on the same logical source line by an
 \grammarterm{identifier},
 \tcode{:}, or
 \tcode{;}
 preprocessing token, or
 
 \item
 an \keyword{export} preprocessing token
-immediately followed on the same logical line by
+immediately followed on the same logical source line by
 one of the two preceding forms.
 \end{itemize}