Use std::string and unique_xmalloc_ptr in compile/ code

Use std::string and unique_xmalloc_ptr in compile/ code

Change various things in the compile/ code to use std::string or
unique_xmalloc_ptr as appropriate.  This allows the removal of some
cleanups.

ChangeLog
2017-09-03  Tom Tromey  <tom@tromey.com>

	* compile/compile.c (compile_register_name_mangled): Return
	std::string.
	* compile/compile-loc2c.c (pushf_register_address): Update.
	(pushf_register): Update.
	* compile/compile-c-types.c (convert_array): Update.
	* compile/compile-c-symbols.c (generate_vla_size): Update.
	(error_symbol_once): Use a gdb::unique_xmalloc_ptr.
	(symbol_substitution_name): Return a gdb::unique_xmalloc_ptr.
	(convert_one_symbol): Update.
	(generate_c_for_for_one_variable): Update.
	* compile/compile-c-support.c (c_get_range_decl_name): Return a
	std::string.
	(generate_register_struct): Update.
	* compile/compile-internal.h (c_get_range_decl_name): Return a
	std::string.
	(compile_register_name_mangled): Return std::string.