PR19260: Teach doxygen to spell correctly the include paths.

Currently we have #include <Sema.h> in the doxygen page documenting Sema. The
patch changes it ot #include "clang/Sema/Sema.h" which is what we would spell if
we need to include it in a real codebase.

Patch by Yuka Takahashi (D32113)!

llvm-svn: 300825