Fix a couple of cornercases in FileSpec + tests

Summary:
This fixes a couple of corner cases in FileSpec, related to AppendPathComponent and
handling of root directory (/) file spec. I add a bunch of unit tests for the new behavior.

Summary of changes:
FileSpec("/bar").GetCString(): before "//bar", after "/bar".
FileSpec("/").CopyByAppendingPathComponent("bar").GetCString(): before "//bar", after "/bar".
FileSpec("C:", ePathSyntaxWindows).CopyByAppendingPathComponent("bar").GetCString(): before "C:/bar", after "C:\bar".

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18044

llvm-svn: 263207