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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@119131 91177308-0d34-0410-b5e6-96231b3b80d8