IntelLLVM: Add support for Intel LLVM-based compilers

Using a single ID 'IntelLLVM' for the suite of Intel compilers based on
the LLVM backend.  The 'IntelLLVM' ID are used for C, C++, and Fortran.
Data Parallel C++ will be handled in a separate commit.

The C and C++ definitions are based on the Clang definitions.  The Intel
LLVM-based C and C++ compilers are based on the Clang front end, so
existing Clang options are more likely to be a good match than options
for the older Intel compilers.

Fortran is based on the older Fortran front end with the LLVM backend.
It has a similar interface to the older versions, though many options
are shared with the C and C++ compilers.

Fixes: #21561
Signed-off-by: William R. Dieter <william.r.dieter@intel.com>