Prevent GetAugmentedArchSpec() from attaching "unknown" environments

Environments are optional and a missing environment is distinct from
the default "unknown" environment enumerator.  The test is negative,
because the function uses the host triple and is unpredictable.

rdar://91007207

https://reviews.llvm.org/D122946

Differential Revision: https://reviews.llvm.org/D122946