Move line_header_hash to dwarf2_per_objfile

Move line_header_hash to dwarf2_per_objfile

The `line_header_hash` field of `struct dwarf2_per_bfd` contains some
`struct line_header` objects.  A `struct line_header` objects contains
some `file_entry` objects.  A `file_entry` object contains a pointer to
the `symtab` object created from it.  The `line_header_hash` is
therefore ultimately objfile-dependent and can't be shared as-is between
objfiles.

Move it from `dwarf2_per_bfd` to `dwarf2_per_objfile`.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_bfd) <line_header_hash>: Move
	to...
	(struct dwarf2_per_objfile) <line_header_hash>: ... here.
	* dwarf2/read.c (handle_DW_AT_stmt_list): Update.

Change-Id: I8d2ee04df4f4847c2db99061fc976c35af98ac71