Remove some manual memory management from compile interface

Remove some manual memory management from compile interface

This changes gdb's compile code to use std::vector in a couple of
places, rather than manual memory management.

gdb/ChangeLog
2020-09-23  Tom Tromey  <tom@tromey.com>

	* compile/compile-cplus-types.c
	(compile_cplus_convert_struct_or_union): Use std::vector.
	(compile_cplus_convert_func): Likewise.
	* compile/compile-c-types.c (convert_func): Use std::vector.