Fix lookup of separate debug file on MS-Windows.

Fix lookup of separate debug file on MS-Windows.

If you put the separate debug file in a global debug directory, GDB on
MS-Windows would fail to find it.  This happens because we obtain the
directory to look up the debug file by concatenating the debug
directory name with the leading directories of the executable, and the
latter includes the drive letter on MS-Windows.  So we get an invalid
file name like

   d:/usr/lib/debug/d:/usr/bin/foo.debug

This commit fixes that by removing the colon of the drive letter,
thus producing

   d:/usr/lib/debug/d/usr/bin/foo.debug

gdb/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* symfile.c (find_separate_debug_file): Remove colon from the
	drive spec of DOS/Windows file names of the target, so that the
	file name produced from DEBUGDIR and the target's directory will
	be valid on DOS/Windows systems.

gdb/doc/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* gdb.texinfo (Separate Debug Files): Document how the
	subdirectory of the global debug directory is computed on
	MS-Windows/MS-DOS.