Use the most recent previous decl to check if inline is added after a definition

This affects this test case:
  void foo();
  template <typename T> class C {
    friend inline void foo();
  };
  inline void foo() {}
  C<int> c;

Here, we instantiate the foo friend decl and add it to foo's redecl
chain. However, our previous decl pointer happens to reference the first
declaration of foo, which is not marked inline. When we check to see if
foo was already defined, we implicitly search all previous decls. We
should do the same for the inline check, instead of just checking this
particular previous decl.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D8872

llvm-svn: 234374