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

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'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107008 91177308-0d34-0410-b5e6-96231b3b80d8