.bazelrc: Add ASan config

[smcallis]

Enables ASAN builds with e.g. bazel -c opt --config=asan

[jmr]

Adding this to bazel.yml would be good, too, but can also be done separately.

[jmr]

A lot of these seem orthogonal and part of -c dbg.

[smcallis]

You can theoretically use this with -c opt or -c fastbuild too, it's probably wise to override them because if you have e.g. -O3 on and the compiler elides problematic reads/writes then you'll get false negatives.