[hwasan] implement free_checks_tail_magic=1

Summary:
With free_checks_tail_magic=1 (default) HWASAN
writes magic bytes to the tail of every heap allocation
(last bytes of the last granule, if the last granule is not fully used)
and checks these bytes on free().

This feature will detect buffer overwires within the last granule
at the time of free().

This is an alternative to malloc_align_right=[1289] that should have
fewer compatibility issues. It is also weaker since it doesn't
detect read overflows and reports bugs at free() instead of at access.

Reviewers: eugenis

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D54656

llvm-svn: 347116