[irgen] Make HasAArch64TBI an IRGen option instead of a LangOpts.

With some changes that I am making, we will no longer need this flag at the SIL
level, so we can just make it an IRGen flag (which it really should have been
in the first place).

lib/Frontend/CompilerInvocation.cpp

@@ -375,7 +375,7 @@ void CompilerInvocation::computeAArch64TBIOptions() {
   auto &LLVMArgs = getFrontendOptions().LLVMArgs;
   auto aarch64_use_tbi =
       std::find(LLVMArgs.begin(), LLVMArgs.end(), "-aarch64-use-tbi");
-  LangOpts.HasAArch64TBI = aarch64_use_tbi != LLVMArgs.end();
+  IRGenOpts.HasAArch64TBI = aarch64_use_tbi != LLVMArgs.end();
 }
 
 void CompilerInvocation::computeCXXStdlibOptions() {