Convert xcoffread.c to type-safe registry API

Convert xcoffread.c to type-safe registry API

This changes xcoffread.c to use the type-safe registry API.  It also
renames coff_symfile_info to xcoff_symfile_info, to avoid any possible
ODR violation.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (struct xcoff_symfile_info): Rename from
	coff_symfile_info.  Add initializers.
	(xcoff_objfile_data_key): Move lower.  Change type.
	(XCOFF_DATA): Rewrite.
	(xcoff_free_info): Remove.
	(xcoff_symfile_init, _initialize_xcoffread, read_xcoff_symtab)
	(read_symbol, read_symbol_lineno, find_linenos, init_stringtab)
	(xcoff_initial_scan): Update.