Move more code to line-header.c

Move more code to line-header.c

This moves some more code out of read.c and into line-header.c.
dwarf_decode_line_header is split into two -- the part remaining in
read.c handles interfacing to the dwarf2_cu; while the part in
line-header.c (more or less) purely handles the actual decoding.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/line-header.h (dwarf_decode_line_header): Declare.
	* dwarf2/read.c
	(dwarf2_statement_list_fits_in_line_number_section_complaint):
	Move to line-header.c.
	(read_checked_initial_length_and_offset, read_formatted_entries):
	Likewise.
	(dwarf_decode_line_header): Split into two.
	* dwarf2/line-header.c
	(dwarf2_statement_list_fits_in_line_number_section_complaint):
	Move from read.c.
	(read_checked_initial_length_and_offset, read_formatted_entries):
	Likewise.
	(dwarf_decode_line_header): New function, split from read.c.