[opt-viewer] Add progress indicators (PR33522)

Summary:
Provide feedback to users of opt-diff.py, opt-stats.py, and opt-viewer.py,
on how many YAML files have finished being processed, and how many HTML
files have been generated. This feedback is particularly helpful for
opt-viewer.py, which may take a long time to complete when given many
large YAML files as input.

The progress indicators use simple output such as the following:

```
Reading YAML files...
    9 of 1197
```

Test plan:
Run `utils/opt-viewer/opt-*.py` on a CentOS and macOS machine, using
Python 3.4 and Python 2.7 respectively, and ensure the output is
formatted well on both.

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: simon.f.whittaker, llvm-commits

Differential Revision: https://reviews.llvm.org/D34735

llvm-svn: 306726