Change dwarf2_per_objfile::line_header_hash to htab_up

This changes dwarf2_per_objfile::line_header_hash to be an htab_up,
and changes it to use heap allocation.

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

	* dwarf2/read.c (~dwarf2_per_objfile): Don't delete
	line_header_hash.
	(handle_DW_AT_stmt_list): Update.  Don't allocate on obstack.
	* dwarf2/read.h (struct dwarf2_per_objfile) <line_header_hash>:
	Change type to htab_up.

Change-Id: Icb148a270838c0f96f38fc4a28b5b77d067927b6