WIP: Make too-many-arguments errors span fn header only

[ghost]

The HIR API has no span for just the function header. This commit handles that
by using the entire function span until the start of the body expression. This
isn't perfect because it includes spaces after the header.

Closes #2488

[ghost]

I need feedback on this. Is this PR worth merging as is? How can I remove the trailing space from the span?

[oli-obk]

I think we should delay fixing this until rust-lang/rust#49154 has been merged. This should allow us to only point to the name of the function.

How can I remove the trailing space from the span?

You'd need to fetch the text snippet, count the trailing whitespace and shorten the span by that count. But I'm fairly confident that the "all idents have spans" PR will get merged in the next few weeks, so we could hold off until then.

Your PR can then be adjusted to make use of the idents.

On the other hand we could just merge the PR now and do that fix later. What do you think?

[ghost]

Let's wait. I will close this and submit a new PR later.