[clang] [clang][Diagnostic] Clarify error message for auto (PR #149781)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think we'd want to extract the determination of a dialog location into a separate function (lambda?) to help calculate it, in both of the diag cases, including hte `err_template_outside_namespace_or_class_scope` diag.

Also: we need a test for this.  A simple reproducer that uses `//expected-error` lines that point to a line instead of 'in general' i think will work.

https://github.com/llvm/llvm-project/pull/149781