Ensure path::iterator and PathParser share the same enumeration values.

To avoid exposing implementation details, path::iterator and PathParser
both implicitly used the same set of values to represent the state,
but they were defined twice. This could have lead to a mismatch
occuring.

This patch moves all of the parser state values into the filesystem
header and changes PathParser to use those value to avoid this.

llvm-svn: 337883