Move DWARF line_header to new file

Move DWARF line_header to new file

This moves the line_header class to a pair of new files, making
dwarf2/read.c somewhat smaller.

2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (dwarf_line_debug): Declare.
	* Makefile.in (COMMON_SFILES): Add dwarf2/line-header.c.
	* dwarf2/read.c: Move line_header code to new files.
	(dwarf_line_debug): No longer static.
	* dwarf2/line-header.c: New file.
	* dwarf2/line-header.h: New file.

Change-Id: I8d9d8a2398b4e888e20cc5dd68d041c28b5a06e3