[Linker] Remove warning when linking ARM and Thumb IR modules.

Summary:
This patch updates Triple::isCompatibleWith to make armxx and thumbxx
triples compatible, as long as the subarch, vendor, os, envorionment and
object format match. Thumb/ARM code generation should be controlled
using the thumb-mode per-function target feature rather than by the
triple to allow mixing Thumb and ARM functions.

D33448 updates Clang's codegen to add thumb-mode for all functions with
armxx or thumbxx triples.

Reviewers: echristo, t.p.northover, rafael, kristof.beyls, rengolin, tejohnson

Reviewed By: tejohnson

Subscribers: rinon, eugenis, pcc, srhines, aemerson, mehdi_amini, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D33287

llvm-svn: 304884