Return unique_xmalloc_ptr from macro scope functions

Return unique_xmalloc_ptr from macro scope functions

This changes the macro scope functions (sal_macro_scope,
user_macro_scope, and default_macro_scope) to return a
unique_xmalloc_ptr, then fixes up the users.  This allowed for the
removal of several cleanups.

2018-02-08  Tom Tromey  <tom@tromey.com>

	* symtab.c (default_collect_symbol_completion_matches_break_on):
	Use unique_xmalloc_ptr.
	* macroscope.h: (sal_macro_scope, user_macro_scope)
	(default_macro_scope): Return unique_xmalloc_ptr.
	* macroscope.c (sal_macro_scope, user_macro_scope)
	(default_macro_scope): Return unique_xmalloc_ptr.
	* macroexp.h (macro_expand, macro_expand_once): Return
	unique_xmalloc_ptr.
	* macroexp.c (macro_expand, macro_expand_once): Return
	unique_xmalloc_ptr.
	* macrocmd.c (macro_expand_command, macro_expand_once_command)
	(info_macro_command, info_macros_command): Use
	unique_xmalloc_ptr.
	* compile/compile-c-support.c (write_macro_definitions): Use
	unique_xmalloc_ptr.
	* c-exp.y (c_parse): Use unique_xmalloc_ptr.