allocate MultiKeywordSelector's out of a bump pointer allocator instead of malloc.

This has two advantages 1) no more leaking them, 2) fewer calls to malloc.

This changes us from calling malloc 3685/1390/883/2974/1185 times respectively on
16/20/24/28/32 byte objects when parsing cocoa.h with pth and -disable-free to
calling it 2816/1020/702/2903/1168 times each respectively.

llvm-svn: 66017