Clean up some names and fix the handling of default sysroots on Windows and

other platforms where the textual default of '/' isn't the system's root
directory. We should probably still make the textual default platform specific,
but this should avoid the particularly bad problem with the previous state: we
applied a sysroot of '/' to '/usr/local/google' which added
'//usr/local/include' to the windows header search path, a share on another
machine named 'usr'. Oops.

llvm-svn: 119131