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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232785 91177308-0d34-0410-b5e6-96231b3b80d8