Change dbxread.c to use type-safe registry

Change dbxread.c to use type-safe registry

This changes dbxread.c to use the type-safe registry.  In a couple of
spots, you'll see that dbx_objfile_data_key.emplace is called but the
result is not used; this is because those functions refer to the key
via the various DBX_* macros.

2019-07-10  Tom Tromey  <tromey@adacore.com>

	* gdb-stabs.h (struct dbx_symfile_info): Add initializers and
	destructor.
	(dbx_objfile_data_key): Change type and declare later.
	(DBX_SYMFILE_INFO): Rewrite.
	* dbxread.c (dbx_objfile_data_key): Change type.
	(dbx_symfile_init): Update.
	(~dbx_symfile_info): Rename from dbx_free_symfile_info.  Update.
	(coffstab_build_psymtabs, elfstab_build_psymtabs)
	(stabsect_build_psymtabs, _initialize_dbxread): Update.