[HWASAN] Modified __hwasan::Thread::unique_id_ to be u32 instead of u64.

This is needed for LSAN integration work. LSAN uses context to store thread_id << 32 + stack_id. In order to fit both we would need thread_id to be 32 bit.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D139060