Auto merge of #3700 - phansch:would_you_like_some_help_with_this_const_fn, r=oli-obk

Prevent incorrect cast_lossless suggestion in const_fn

`::from` is not a const fn, so applying the suggestion of
`cast_lossless` would fail to compile. The fix is to skip the lint if
the cast is found inside a const fn.

Fixes #3656

diff --cc clippy_lints/src/utils/mod.rs
Simple merge