Improve the Python bindings for libclang in a few ways, from Eli

Bendersky. Specifically: 

* Implemented a new function in libclang: clang_isAttribute

* Fixing TranslationUnit.get_includes to only go through the argument
* buffer when it contains something. This fixed a crash on Windows 

* clang_getFileName returns CXString, not char*. Made appropriate
* fixes in cindex.py - now the relevant tests pass and we can see the
* full locations correctly again (previously there was garbage in
* place of the file name) 
* Exposed clang_getCursorDisplayName to the python bindings

llvm-svn: 134460