Teach libclang to walk the base and member initializers of a

Teach libclang to walk the base and member initializers of a
constructor, in source order. Also introduces a new reference kind for
class members, which is used here (for member initializers) and will
also be used for designated initializers and offsetof.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@113545 91177308-0d34-0410-b5e6-96231b3b80d8