WPXSvInputStreamImpl was hammering on getFilePos pretty hard, and
getFilePos uses a mutex, which is slow when it is called from every
single read. So switch to using std::atomic to access position.
This is specifically fixing the performance of queryTypeByDescriptor
when called from a basic macro on a local test file.
This takes my test macro from 8s to 4s.
Reviewed-by: Noel Grandin <firstname.lastname@example.org>