[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275894 91177308-0d34-0410-b5e6-96231b3b80d8