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

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

Compute an effective target triple exactly once in ConstructJob(), and
then simply pass around references to it. This eliminates wasteful
re-computation of effective triples (e.g in getARMFloatABI()).

Differential Revision: https://reviews.llvm.org/D22290

llvm-svn: 275895