Replace string GNU Triples with llvm::Triple in MCSubtargetInfo and create*MCSubtarge...

Replace string GNU Triples with llvm::Triple in MCSubtargetInfo and create*MCSubtargetInfo(). NFC.

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

Reviewers: rafael

Reviewed By: rafael

Subscribers: rafael, ted, jfb, llvm-commits, rengolin, jholewinski

Differential Revision: http://reviews.llvm.org/D10311

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239467 91177308-0d34-0410-b5e6-96231b3b80d8