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)!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300825 91177308-0d34-0410-b5e6-96231b3b80d8