[CLOpt] replace uses of Str with Core.String
[CLOpt] replace uses of Str with Core.String

Summary:
Running `infer --help` with spacetime showed that `Str` was a hog. And indeed perf is 74% better now:

```
before:
bash -c 'for i in $(seq 100); do infer --help > /dev/null 2>/dev/null; done'  28.02s user 0.62s system 98% cpu 29.205 total
after:
bash -c 'for i in $(seq 100); do infer --help > /dev/null 2>/dev/null; done'  6.34s user 0.64s system 92% cpu 7.557 total
```

Running on many files one after the other is now 64% faster too:
```
$ cd infer/tests/codetoanalyze/c/errors
$ # before
$ rm -fr infer-out && time bash -c 'for src in */*.c; do infer -a capture --continue -- clang -c $src; done'
bash -c   7.77s user 0.66s system 97% cpu 8.647 total
$ # after
$ rm -fr infer-out && time bash -c 'for src in */*.c; do infer -a capture --continue -- clang -c $src; done'
bash -c   2.35s user 0.56s system 93% cpu 3.119 total
$ time infer -a capture -- clang -c */*.c
infer -a capture -- clang -c */*.c  0.54s user 0.20s system 99% cpu 0.737 total
```

Reviewed By: mbouaziz

Differential Revision: D4875803

fbshipit-source-id: cfcfa69

facebook/infer,jvillard/infer,blzq/infer,blzq/infer,ropas/infer,ropas/infer,jsachs/infer,facebook/infer,jvillard/infer,ropas/infer,jvillard/infer,facebook/infer,jvillard/infer,blzq/infer,blzq/infer,blzq/infer,ropas/infer,jsachs/infer,ryoon/infer,jvillard/infer,ryoon/infer,facebook/infer,ryoon/infer,ryoon/infer,jvillard/infer,facebook/infer,jsachs/infer,jsachs/infer,facebook/infer,jvillard/infer,facebook/infer,ropas/infer,ryoon/infer,blzq/infer,ropas/infer,jsachs/infer,ryoon/infer,jsachs/infer