[hwasan] Don't report short-granule shadow as overwritten.

The shadow for a short granule is stored in the last byte of the
granule. Currently, if there's a tail-overwrite report (a
buffer-overflow-write in uninstrumented code), we report the shadow byte
as a mismatch against the magic.

Fix this bug by slapping the shadow into the expected value. This also
makes sure that if the uninstrumented WRITE does clobber the shadow
byte, it reports the shadow was actually clobbered as well.

Reviewed By: eugenis, fmayer

Differential Revision: https://reviews.llvm.org/D107938