[CodeGen] Print performance counter information in CSV.

This ensures that tools can parse performance information which Polly
generates easily.

- Sample output:
```name=out.csv
scop function, entry block name, exit block name, total time
warmup, %entry.split, %polly.merge_new_and_old, 1960
f, %entry.split, %polly.merge_new_and_old, 1238
g, %entry.split, %polly.merge_new_and_old, 1218
```

- Example code to parse output:
```lang=python, name=example-parse.py
import asciitable
import sys

table = asciitable.read('out.csv', delimiter=',')
asciitable.write(table, sys.stdout, delimiter=',')
```

llvm-svn: 304533