Autogenerate the shebang lines for tools/opt-viewer

Summary:
Since these files depend on the built python modules, they need to use
the right python binary to run them. So use configure_file
to set the right shebang line.

Patch By: cbiesinger (Christian Biesinger)

Reviewers: chandlerc, beanz, anemet

Reviewed By: anemet

Subscribers: compnerd, JDevlieghere, mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65983

llvm-svn: 369486