[hwasan] Verify Android TLS slot at startup.

Summary:
Add a check that TLS_SLOT_TSAN / TLS_SLOT_SANITIZER, whichever
android_get_tls_slot is using, is not conflicting with
TLS_SLOT_DLERROR.

Reviewers: rprichard, vitalybuka

Subscribers: srhines, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D55587

llvm-svn: 348979