[HWASan] Remove incorrect unreachable.

[fmayer]

Summary

This function could be called wih access_info & 0x20 or with
flags()->halt_on_error, in which case HandleTagMismatch returns (is not
fatal).

[eugenis]

LGTM

compiler-rt/lib/hwasan/hwasan.cpp

void HwasanTagMismatch(uptr addr, uptr access_info, uptr *registers_frame,
   ai.addr = addr;
   if ((access_info & 0xf) == 0xf)
     ai.size = outsize;
   else
     ai.size = 1 << (access_info & 0xf);
 
   HandleTagMismatch(ai, (uptr)__builtin_return_address(0),
                     (uptr)__builtin_frame_address(0), nullptr, registers_frame);
-  __builtin_unreachable();
 }
 
 Thread *GetCurrentThread() {
   uptr *ThreadLongPtr = GetCurrentThreadLongPtr();
   if (UNLIKELY(*ThreadLongPtr == 0))
     return nullptr;
   auto *R = (StackAllocationsRingBuffer *)ThreadLongPtr;
   return hwasanThreadList().GetThreadByBufferAddress((uptr)R->Next());