ArchSpec: fix unintentional promotion of unspecified unknowns to specified unknowns

* ArchSpec::MergeFrom() would erroneously promote an unspecified
  unknown to a specified unknown when both the ArchSpec and the merged
  in ArchSpec were both unspecified unknowns. This no longer happens,
  which fixes issues with global module cache lookup in some
  situations.

* Added ArchSpec::DumpTriple(Stream&) that now properly prints
  unspecified unknowns as '*' and specified unknows as 'unknown'.
  This makes it trivial to tell the difference between the two.
  Converted printing code over ot using DumpTriple() rather than
  building from scratch.

* Fixed up a couple places that were not guaranteeing that an
  unspecified unknown was recorded as such.

llvm-svn: 250253