gdb: make macro_expand_next return a gdb::unique_xmalloc_ptr<char>

For some reason, macro_expand_next does not return a
gdb::unique_xmalloc_ptr<char>, like its counterparts macro_expand and
macro_expand_once.  This patch fixes that.

macro_buffer::release now returns a gdb::unique_xmalloc_ptr<char> too,
which required updating the other callers.  The `.release (). release
()` in macro_stringify looks a bit funny, but it's because one release
is for the macro_buffer, and the other is for the unique ptr.

I removed the ATTRIBUTE_UNUSED_RESULT on macro_buffer::release, I don't
really understand why it's there.  I don't see how this method could be
called without using the result, that would be an obvious memory leak.
The commit that introduced it (4e4a8b93 "Add ATTRIBUTE_UNUSED_RESULT
to macro_buffer") doesn't give any details.

gdb/ChangeLog:

	* c-exp.y (scan_macro_expansion): Don't free `expansion`.
	(lex_one_token): Update.
	* macroexp.c (struct macro_buffer) <release>: Return
	gdb::unique_xmalloc_ptr<char>.
	(macro_stringify): Update.
	(macro_expand): Update.
	(macro_expand_next): Return gdb::unique_xmalloc_ptr<char>.
	* macroexp.h (macro_expand_next): Likewise.

Change-Id: I67a74d0d479d2c20cdc82161ead7c54cea034f56