[Hexagon] Mark raise_relocation_error as NORETURN.

[fhahn]

Summary

This silences a couple of implicit fallthrough warnings with GCC 7.1 in
this file.

[kparzysz]

LGTM, thanks.

lib/Target/Hexagon/MCTargetDesc/HexagonMCCodeEmitter.cpp

if (MI.getOpcode() >= Hexagon::DuplexIClass0 &&
 
     Binary |= subInstSlot0Bits | (subInstSlot1Bits << 16);
   }
   support::endian::Writer<support::little>(OS).write<uint32_t>(Binary);
   ++MCNumEmitted;
 }
 
 namespace {
+LLVM_ATTRIBUTE_NORETURN
 void raise_relocation_error(unsigned bits, unsigned kind) {
   std::string Text;
   {
     llvm::raw_string_ostream Stream(Text);
     Stream << "Unrecognized relocation combination bits: " << bits
            << " kind: " << kind;
   }
   report_fatal_error(Text);