[lit] Add %:[STpst] to represent paths without colons on Windows.

Summary:
We need these variables to concatenate two absolute paths to construct
a valid path. Currently, %t\%t is, for example, expanded to C:\foo\C:\foo,
which is not a valid path because ":" is not a valid path character
on Windows. With this patch, %t will be expanded to C\foo.

Differential Revision: http://reviews.llvm.org/D19757

llvm-svn: 268168