Warn about self references in in-class initializers.

This makes Clang warn about self references in in-class initializers,
for example:

  struct S {
    int a = a + 42;
  };

This basically just moves UninitializedFieldVisitor up a bit in
SemaDeclCXX.cpp, and adds a call to it from ActOnCXXInClassMemberInitializer.

llvm-svn: 164131