[hwasan] optionally right-align heap allocations

Summary:
... so that we can find intra-granule buffer overflows.
The default is still to always align left.
It remains to be seen wether we can enable this mode at scale.

Reviewers: eugenis

Reviewed By: eugenis

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

Differential Revision: https://reviews.llvm.org/D53789

llvm-svn: 347082