[mlgo] Use LLVM_HAVE_TFLITE instead of LLVM_HAVE_TF_API in C++ code (NFC)

We use LLVM_HAVE_TFLITE as the key to enable the mlgo work these days,
and LLVM_HAVE_TF_API is defined whenever LLVM_HAVE_TF_API is defined.

I'm posting this patch because it's purely mechanical.

I'll post a follow-up patch to remove LLVM_HAVE_TF_API in non-C++
files, and that will not be as mechanical as this one.

Differential Revision: https://reviews.llvm.org/D139863