[clang] Additional FP classification functions (#69041)

C language standard defined library functions `iszero`, `issignaling`
and `issubnormal`, which did not have counterparts among clang builtin
functions. This change adds new functions:

    __builtin_iszero
    __builtin_issubnormal
    __builtin_issignaling

They provide builtin implementation for the missing standard functions.

Pull request: https://github.com/llvm/llvm-project/pull/69041