[Driver] Rename DefaultTargetTriple to TargetTriple

While this value is initialized with the DefaultTargetTriple, it
can be later overriden using the -target flag so TargetTriple is
a more accurate name. This change also provides an accessor which
could be accessed from ToolChain implementations.

Differential Revision: https://reviews.llvm.org/D47357

llvm-svn: 333468