python bindings: Add support for translationUnit.reparse().

This is the first step to make the clang_complete vim plugin work with
libclang. Reparsing improves parsing time from 0.8 to 0.25 secs for
one of my LLVM .cpp files.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124956 91177308-0d34-0410-b5e6-96231b3b80d8