[opt-viewer] Use Python 3-compatible `intern()`

Summary:
In Python 2, `intern()` is a builtin function available to all programs.
In Python 3, it was moved into the `sys` module, available as
`sys.intern`. Import it such that, within `optrecord.py`, `intern()` is
available whether run using Python 2 or 3.

Test Plan:
Run `opt-viewer.py` using Python 3, confirm it no longer
encounters a runtime error when `intern()` is called.

Reviewers: anemet

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36622

llvm-svn: 310739