Bug 1114662 - add --enable-thread-sanitizer configure option; r=glandium

Like --enable-{address,memory}-sanitizer, this doesn't add the
appropriate -fsanitize=FOO flags; it merely sets internal defines and
the LLVM_SYMBOLIZER variable.
Bug 1114684 - inform mozinfo about the MOZ_TSAN build configuration; r=ahal

Various bits of the test harnesses key off of mozinfo.info.get('asan');
we will need a similar switch for finding out whether this build
supports tsan.
Bug 1114745 - remove dead IS_ASAN defines from automation-build.mk; r=ted

Nothing uses IS_ASAN, likely because all relevant uses have been ported
over to mozinfo.