Replace string GNU Triples with llvm::Triple in TargetMachine. NFC.

Replace string GNU Triples with llvm::Triple in TargetMachine. NFC.

Summary:
For the moment, TargetMachine::getTargetTriple() still returns a StringRef.

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

Reviewers: rengolin

Reviewed By: rengolin

Subscribers: ted, llvm-commits, rengolin, jholewinski

Differential Revision: http://reviews.llvm.org/D10362

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239554 91177308-0d34-0410-b5e6-96231b3b80d8