Allow autotools to build and install cpp11-migrate

This patch fixes bug15793
cpp11-migrate was built but not installed (make install)
in the autotools build.
Revert "revert r179735, it has no testcases, and doesn't really make sense."

This un-reverts r179735 and reverts commit r180574.

This fixes assertion failures for me locally and should fix the failures
on Windows reported widely on llvm-dev.  We should check if the bots
caught this and if so why not.