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

Summary:
This affects other tools so the previous C++ API has been retained as a
deprecated function for the moment. Clang has been updated with a trivial
patch (not covered by the pre-commit review) to avoid breaking -Werror builds.
Other in-tree tools will be fixed with similar trivial patches.

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: llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D10366

llvm-svn: 239721