[CodeComplete] Do not complete self-initializations

Summary:
Removes references to initialized variable from the following completions:

    int x = ^;

Handles only the trivial cases where the variable name is completed
immediately at the start of initializer or assignment, more complicated
cases aren't covered, e.g. these completions still contain 'x':

    // More complicated expressions.
    int x = foo(^);
    int x = 10 + ^;
    // Other kinds of initialization.
    int x{^};
    int x(^);
    // Constructor initializers.
    struct Foo {
      Foo() : x(^) {}
      int x;
    };

We should address those in the future, but they are outside of the scope of
this initial change.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D54156

llvm-svn: 346301