funcs: Don't panic if templatefile path is sensitive

[apparentlymart]

Previously we were partially propagating any marks from the path, but not going all the way so we still ran into trouble when trying to use the string containing the file contents.

Now we'll have loadTmpl also return the marks it had to read through to actually parse the template, and then we'll use those (instead of the original path marks) to mark the result. In practice the pathMarks and the tmplMarks should always match today, but this is intentionally structured to make the data flow clearer -- the marks always travel along with whatever they related to -- so we're less likely to break this accidentally under future maintenance.

This fixes https://github.com/hashicorp/terraform/issues/31119.