Auto merge of #47203 - varkor:output-filename-conflicts-with-directory, r=estebank

Warn when rustc output conflicts with existing directories

When the compiled executable would conflict with a directory, display a
rustc error instead of a verbose and potentially-confusing linker
error. This is a usability improvement, and doesn’t actually change
behaviour with regards to compilation success. This addresses the
concern in #35887. Fixes #13098.

diff --cc src/librustc/session/config.rs
Simple merge