libcpp: move line typedef and column-numbering comment to top of file

The description of our 1-based column-numbering convention was in
a non-obvious place withn line-map.h; this patch moves it to the top
of that header.

libcpp/ChangeLog:
	* include/line-map.h (linenum_type): Move this typedef and the
	comment describing column numbering to near the top of the file.

From-SVN: r254703

libcpp/ChangeLog

+2017-11-13  David Malcolm  <dmalcolm@redhat.com>
+
+	* include/line-map.h (linenum_type): Move this typedef and the
+	comment describing column numbering to near the top of the file.
+
 2017-11-06  Mukesh Kapoor  <mukesh.kapoor@oracle.com>
 
 	PR c++/80955

libcpp/include/line-map.h

@@ -26,6 +26,29 @@ along with this program; see the file COPYING3.  If not see
 #define GTY(x) /* nothing */
 #endif
 
+/* Both gcc and emacs number source *lines* starting at 1, but
+   they have differing conventions for *columns*.
+
+   GCC uses a 1-based convention for source columns,
+   whereas Emacs's M-x column-number-mode uses a 0-based convention.
+
+   For example, an error in the initial, left-hand
+   column of source line 3 is reported by GCC as:
+
+      some-file.c:3:1: error: ...etc...
+
+   On navigating to the location of that error in Emacs
+   (e.g. via "next-error"),
+   the locus is reported in the Mode Line
+   (assuming M-x column-number-mode) as:
+
+     some-file.c   10%   (3, 0)
+
+   i.e. "3:1:" in GCC corresponds to "(3, 0)" in Emacs.  */
+
+/* The type of line numbers.  */
+typedef unsigned int linenum_type;
+
 /* Reason for creating a new line map with linemap_add.  LC_ENTER is
    when including a new file, e.g. a #include directive in C.
    LC_LEAVE is when reaching a file's end.  LC_RENAME is when a file
@@ -43,9 +66,6 @@ enum lc_reason
   /* FIXME: add support for stringize and paste.  */
 };
 
-/* The type of line numbers.  */
-typedef unsigned int linenum_type;
-
 /* The typedef "source_location" is a key within the location database,
    identifying a source location or macro expansion, along with range
    information, and (optionally) a pointer for use by gcc.
@@ -1251,26 +1271,6 @@ typedef struct
   bool sysp;
 } expanded_location;
 
-/* Both gcc and emacs number source *lines* starting at 1, but
-   they have differing conventions for *columns*.
-
-   GCC uses a 1-based convention for source columns,
-   whereas Emacs's M-x column-number-mode uses a 0-based convention.
-
-   For example, an error in the initial, left-hand
-   column of source line 3 is reported by GCC as:
-
-      some-file.c:3:1: error: ...etc...
-
-   On navigating to the location of that error in Emacs
-   (e.g. via "next-error"),
-   the locus is reported in the Mode Line
-   (assuming M-x column-number-mode) as:
-
-     some-file.c   10%   (3, 0)
-
-   i.e. "3:1:" in GCC corresponds to "(3, 0)" in Emacs.  */
-
 /* A location within a rich_location: a caret&range, with
    the caret potentially flagged for display.  */