[clang-tidy] fix unit tests for dropped _Float16 support in X86

Summary:
Because _Float16 was disabled for X86 targets the unit-tests started failing.
Extract the pieces for _Float16 and run theses tests under AArch64.

Reviewers: aaron.ballman, erichkeane, lebedev.ri

Reviewed By: erichkeane

Subscribers: javed.absar, xazax.hun, kristof.beyls, cfe-commits

Differential Revision: https://reviews.llvm.org/D57249

llvm-svn: 352231