bindings: expose diagnostic formatting to Python

This makes it easier for tools using the Python libclang bindings to display
diagnostics in a manner consistent with clang.

Patch by Omar Sandoval!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@278315 91177308-0d34-0410-b5e6-96231b3b80d8