[Driver] Make Driver::DefaultTargetTriple private (NFCI)

No in-tree targets access this `DefaultTargetTriple` directly, and usage
of default triples is generally discouraged. Make the field private.

This is part of en effort to make the clang driver use effective triples
more pervasively.

Differential Revision: https://reviews.llvm.org/D22289

llvm-svn: 275894