[Index] Use locations to uniquify function-scope BindingDecl USR

Summary:
This makes BindingDecl's of the same name have different USRs, so that references can be correctly attributed.

    int a[1] = {};
    { auto [x] = a; x; }
    { auto [x] = a; x; }

Reviewers: akyrtzi, arphaman, rsmith, hokein

Reviewed By: hokein

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D52445

llvm-svn: 344010