Check if a path is already absolute before trying to make it so.

The FileSystem::makeAbsolute function has been calculating the current
working directory unconditionally, even when it is not needed. This calls
down to llvm::sys::fs::current_path, which is relatively expensive
because it stats two directories, regardless of whether those paths are
already in the stat cache. The net effect is that when using the
VFS, every stat during header search turns into three stats. With this
change, we get back to a single stat for absolute directory paths.

llvm-svn: 264519