[llvm][NFC] expose LLVM_HAVE_TF_API through llvm-config.h

Summary:
This allows users of the llvm library discover whether llvm was built
with the tensorflow c API dependency, which helps if using the TFUtils
wrapper, for example.

We don't do the same for the LLVM_HAVE_TF_AOT flag, because that does
not expose any API.

Reviewers: mehdi_amini, davidxl

Subscribers: mgorny, aaron.ballman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83746