Fix double slash is path

[0xLeif]

https://pubs.opengroup.org/onlinepubs/009695399/

3.266 Pathname

A character string that is used to identify a file. In the context of IEEE Std 1003.1-2001, a pathname consists of, at most, {PATH_MAX} bytes, including the terminating null byte. It has an optional beginning slash, followed by zero or more filenames separated by slashes. A pathname may optionally contain one or more trailing slashes. Multiple successive slashes are considered to be the same as one slash.

[0xLeif]

swiftlang/swift-foundation#974