[polly][Support] Un-break polly tests
Previously, the polly unit tests were stuck in a infinite loop.
There was an edge case in StringRef::count() introduced by 9f6b13e5cce96066d7262d224c971d93c2724795, where an empty 'Str' would cause the function to never exit.
Also fixed usage in polly.

GitOrigin-RevId: 92b68c1937cd065a2fc44d18c1099de7da19b356
1 file changed