[HWASan] Add aliasing flag and enable HWASan to use it.

-fsanitize-hwaddress-experimental-aliasing is intended to distinguish
aliasing mode from LAM mode on x86_64.  check-hwasan is configured
to use aliasing mode while check-hwasan-lam is configured to use LAM
mode.

The current patch doesn't actually do anything differently in the two
modes.  A subsequent patch will actually build the separate runtimes
and use them in each mode.

Currently LAM mode tests must be run in an emulator that
has LAM support.  To ensure LAM mode isn't broken by future patches, I
will next set up a QEMU buildbot to run the HWASan tests in LAM.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D102288