[Utility] Remove Triple{Environment,OS,Vendor}IsUnspecifiedUnknown from ArchSpec

Summary:
These functions should always return the opposite of the
`Triple{Environment,OS,Vendor}WasSpecified` functions. Unspecified unknown is
the same as unspecified, which is why one set of functions should give us what
we want. It's possible to have specified unknown, which is why we can't just
rely on checking the enum values of vendor/os/environment. We must also ensure
that the names of these are empty and not "unknown".

Differential Revision: https://reviews.llvm.org/D58653

llvm-svn: 354933