Fix some handling of AST nodes with diagnostics.

Fix some handling of AST nodes with diagnostics.

The diagnostic system for Clang can already handle many AST nodes.  Instead
of converting them to strings first, just hand the AST node directly to
the diagnostic system and let it handle the output.  Minor changes in some
diagnostic output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328688 91177308-0d34-0410-b5e6-96231b3b80d8