[ASan] Retire mac_ignore_invalid_free, remove some dead code.

mac_ignore_invalid_free was helpful when ASan runtime used to intercept
CFAllocator and sometimes corrupted its memory. This behavior had been long
gone, and the flag was unused.
This patch also deletes ReportMacCfReallocUnknown(), which was used by the
CFAllocator realloc() wrapper.

llvm-svn: 254722