Provide better diagnostic wording for initializers on static

Provide better diagnostic wording for initializers on static
data member definitions when the variable has an initializer
in its declaration.

For the following code:

  struct S {
    static const int x = 42;
  };
  const int S::x = 42;

This patch changes the diagnostic from:

  a.cc:4:14: error: redefinition of 'x'
  const int S::x = 42;
               ^
  a.cc:2:20: note: previous definition is here
    static const int x = 42;
                     ^
to:

  a.cc:4:18: error: static data member 'x' already has an initializer
  const int S::x = 42;
                   ^
  a.cc:2:24: note: previous initialization is here
    static const int x = 42;
                         ^

Differential Revision: http://llvm-reviews.chandlerc.com/D2235

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195306 91177308-0d34-0410-b5e6-96231b3b80d8