Add opt-viewer testing

Detects whether we have the Python modules (pygments, yaml) required by
opt-viewer and hooks this up to REQUIRES.

This fixes https://bugs.llvm.org/show_bug.cgi?id=34129 (the lack of opt-viewer
testing).

It's also related to https://github.com/apple/swift/pull/12938 and the idea is
to expose LLVM_HAVE_OPT_VIEWER_MODULES to the Swift cmake.

Differential Revision: https://reviews.llvm.org/D40202

llvm-svn: 319188