test: Make a start on a test suite for libLTO.

This works in a similar way to the gold plugin tests. We search for a compatible
linker on $PATH and use it to run tests against our just-built libLTO. To start
with, test the just added opt level functionality.

Differential Revision: http://reviews.llvm.org/D8472

llvm-svn: 232785
GitOrigin-RevId: bbdc26ba1b05215347cae02242fb512586a9554e
7 files changed