lit: flesh out `SubsituteCaptures` further
lit: flesh out `SubsituteCaptures` further

Add overloads for `__len__` and `__getitem__` to allow use of this class
on Linux as well as Windows.  With these overloads, lit can be used on
both hosts for the swift testsuite.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@331431 91177308-0d34-0410-b5e6-96231b3b80d8

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