Change dwarf_decode_macro_bytes calling convention

Change dwarf_decode_macro_bytes calling convention

This changes dwarf_decode_macro_bytes to accept a buildsym_compunit
rather than a dwarf2_cu.  This enables some subsequent changes; and
also makes the function accept a "more specific" parameter.

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

	* dwarf2/read.c (macro_start_file): Change "cu" parameter to
	"builder".
	(dwarf_decode_macro_bytes): Likewise.  Add dwarf2_per_objfile
	parameter.
	(dwarf_decode_macros): Update.