Remove custom path manipulation functions from FileSpec

Summary:
now that llvm supports host-agnostic path manipulation functions (and
most of their kinks have been ironed out), we can remove our copies of
the path parsing functions in favour of the llvm ones.

This should be NFC except for the slight difference in handling of the
"//" path, which is now normalized to "/" (this only applies to the
literal "//" path; "//net" and friends still get to keep the two
slashes).

Reviewers: zturner, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D46687

llvm-svn: 332088