Add support for traversing initializer lists (in constructors), which

we ignoring before.  To give access to the names on the initializer,
which aren't a type or an expr or a decl, I've introduced a new
TraverseInitializer.  By default, it just traverses on the expr that
the name is being initialized to.

Reviewed by chandlerc.  Tested via clang's 'make test'.

llvm-svn: 107008