Add support for LLVM 19+ compilers

[climbfuji]

Is your feature request related to a problem? Please describe.

LLVM 19 (clang, clang++, flang-new) has received a lot of attention and praise, in particular because the flang-new compiler apparently offers the most complete Fortran 2018 and 2023 standard implementation of the various Fortran compilers.

Describe the solution you'd like
We should support these compilers. Initially, we can focus on the basic ufs-weather-model-env and neptune-env environments without the many Python packages and their complicated interdependencies, but if the testing with the UFS and NEPTUNE is promising, we want to be able to build the full unified environment.

Additional context

The standalone neptune-env without the Python variant already builds except for an update needed for esmf (spack/spack#48026) and except for ip (see NOAA-EMC/NCEPLIBS-ip#263).