[lex.phases] Make note on the notion of file more prominent

The notion that when the standard refers to files does not
necessarily imply a file in a traditional filing system is
more fundamental than its late appearance in the middle of
phase 7 of translation.  Move that note right to the top of
[lex] where we first talk of storing the program text in
source files, where is will be understood even before the
phases of translation.

Author: AlisdairM

source/lex.tex

@@ -42,6 +42,13 @@
 Translation units can be separately translated and then later linked
 to produce an executable program\iref{basic.link}.
 \end{note}
+\begin{note}
+Source files, translation
+units and translated translation units need not necessarily be stored as
+files, nor need there be any one-to-one correspondence between these
+entities and any external representation. The description is conceptual
+only, and does not specify any particular implementation.
+\end{note}
 \indextext{compilation!separate|)}
 
 \rSec1[lex.phases]{Phases of translation}%
@@ -192,13 +199,6 @@
 dependency\iref{module.unit,module.import}
 are required to be available.
 \begin{note}
-Source files, translation
-units and translated translation units need not necessarily be stored as
-files, nor need there be any one-to-one correspondence between these
-entities and any external representation. The description is conceptual
-only, and does not specify any particular implementation.
-\end{note}
-\begin{note}
 Previously translated translation units can be preserved individually or in libraries.
 The separate translation units of a program communicate\iref{basic.link} by (for example)
 calls to functions whose names have external or module linkage,