Introduce a new, cursor-based traversal function that visits the

children of a given cursor, regardless of what kind of cursor it
is. This is a generalization of clang_loadDeclaration and
clang_loadTranslationUnit that will also extent to statements,
expressions, etc.

As proof-of-concept, switched clang_loadDeclaration() from its own
visitor over to an instance of this traversal function internally.

llvm-svn: 94022