Retry: [Driver] Compute effective target triples once per job (NFCI)

Compute an effective triple once per job. Cache the triple in the
prevailing ToolChain for the duration of the job.

Clients which need effective triples now look them up in the ToolChain.
This eliminates wasteful re-computation of effective triples (e.g in
getARMFloatABI()).

While we're at it, delete MachO::ComputeEffectiveClangTriple. It was a
no-op override.

Differential Revision: https://reviews.llvm.org/D22596

llvm-svn: 276937