Change the last few internal StringRef triples into Triple objects.

Summary:
This concludes the patch series to eliminate StringRef forms of GNU triples
from the internals of LLVM that began in r239036.

At this point, the StringRef-form of GNU Triples should only be used in the
public API (including IR serialization) and a couple objects that directly
interact with the API (most notably the Module class). The next step is to
replace these Triple objects with the TargetTuple object that will represent
our authoratative/unambiguous internal equivalent to GNU Triples.

Reviewers: rengolin

Subscribers: llvm-commits, jholewinski, ted, rengolin

Differential Revision: http://reviews.llvm.org/D10962

llvm-svn: 241472