Make FileSpec handling platform-independent

Summary:
Even though FileSpec attempted to handle both kinds of path syntaxes (posix and windows) on both
platforms, it relied on the llvm path library to do its work, whose behavior differed on
different platforms. This led to subtle differences in FileSpec behavior between platforms. This
replaces the pieces of the llvm library with our own implementations. The functions are simply
copied from llvm, with #ifdefs replaced by runtime checks for ePathSyntaxWindows.

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18689

llvm-svn: 265299