Warn about self references in in-class initializers.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164131 91177308-0d34-0410-b5e6-96231b3b80d8