External Issue: Disallow CHPL_TARGET_COMPILER=clang-included when CHPL_LLVM=system

Notifications Issues
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17598, "BryantLam", "Disallow CHPL_TARGET_COMPILER=clang-included when CHPL_LLVM=system", "2021-04-23T22:14:14Z"

Related #17584 and this post.

As of 1.24.1,

CHPL_LLVM=system
CHPL_TARGET_COMPILER=clang-included`

behaves like (identically to?)

CHPL_LLVM=system
CHPL_TARGET_COMPILER=clang

If CHPL_LLVM=system, I think CHPL_TARGET_COMPILER=clang-included should be an illegal value.