Address -Wsign-compare diagnostics

... which have been introduced in r223152, for example hundreds of:

    [...]/gcc/input.h:37:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

	libcpp/
	* include/line-map.h (RESERVED_LOCATION_COUNT): Change type to
	source_location.

From-SVN: r225558

libcpp/ChangeLog

+2015-07-08  Thomas Schwinge  <thomas@codesourcery.com>
+
+	* include/line-map.h (RESERVED_LOCATION_COUNT): Change type to
+	source_location.
+
 2015-07-02  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR preprocessor/53690

libcpp/include/line-map.h

@@ -870,7 +870,7 @@ bool linemap_location_from_macro_expansion_p (const struct line_maps *,
 /* source_location values from 0 to RESERVED_LOCATION_COUNT-1 will
    be reserved for libcpp user as special values, no token from libcpp
    will contain any of those locations.  */
-const int RESERVED_LOCATION_COUNT = 2;
+const source_location RESERVED_LOCATION_COUNT = 2;
 
 /* Converts a map and a source_location to source line.  */
 inline linenum_type