Fixing find_first_of O(n) case

Fixing find_first_of O(n) case

Summary:
The `memchr()`-based `find_first_of()` behaves extremely badly when it's used in a loop and the input string doesn't contain all the needles. This was discovered when a reasonable line-breaking routine tried to use it to break lines by a mix of '\r' and '\n'. The entire remainder of the file was scanned each time a line was read.

Before:

```
CountDelimsBase                    1.26s   794.86m
CountDelimsNoSSE         100.03%   1.26s   795.12m
CountDelimsStd         40501.63%   3.11ms  321.93
CountDelimsMemchr         98.31%   1.28s   781.41m
CountDelimsByteSet     23162.41%   5.43ms  184.11
```

After:

```
CountDelimsBase                   3.20ms  312.08 <-- Base impl no longer considers memchr
CountDelimsNoSSE         102.37%  3.13ms  319.47
CountDelimsStd           103.19%  3.11ms  322.05
CountDelimsMemchr          0.25%  1.27s   788.39m
CountDelimsByteSet        59.68%  5.37ms  186.27
```

Test Plan: Benchmarks

Reviewed By: njormrod@fb.com

Subscribers: folly-diffs@, yfeldblum

FB internal diff: D1823536

Signature: t1:1823536:1423081687:bb2ec8cdea477ee9b9c8d8ad2bfdecdc52657515