Improve error for "override" + non-virtual func.

Consider something like the following:

struct X {
  virtual void foo(float x);
};
struct Y : X {
  void foo(double x) override;
};

The error is almost certainly that Y::foo() has the wrong signature,
rather than incorrect usage of the override keyword.  This patch
adds an appropriate diagnostic for that case.

Fixes <rdar://problem/14785106>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190109 91177308-0d34-0410-b5e6-96231b3b80d8