[clang] Use llvm.is_fpclass to implement FP classification functions

Builtin floating-point number classification functions:

    - __builtin_isnan,
    - __builtin_isinf,
    - __builtin_finite, and
    - __builtin_isnormal

now are implemented using `llvm.is_fpclass`.

This change makes the target callback `TargetCodeGenInfo::testFPKind`
unneeded. It is preserved in this change and should be removed later.

Differential Revision: https://reviews.llvm.org/D112932