fix: suppress from_over_into when blanket From impl would conflict

[barry3406]

Fixes #16823

When a type has a generic From impl like:

impl<T> From<T> for Foo where String: From<T> { ... }

The from_over_into lint would suggest converting impl Into<String> for Foo to impl From<Foo> for String. But that causes a coherence conflict (E0119) because it makes the blanket impl's where clause satisfiable, which overlaps with core's impl<T> From<T> for T.

Before: lint fires, suggestion doesn't compile

warning: an implementation of `From` is preferred since it gives you `Into<_>` for free
  --> src/lib.rs:13:1
   |
13 | impl Into<String> for Foo {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: replace the `Into` implementation with `From<Foo>`

After: lint correctly suppressed, no false positive

The fix checks whether the self type has any From impl where the source type is a bare generic parameter (like impl<T> From<T> for Foo). If so, the lint is skipped.

Added a test case with the exact reproducer from the issue.

changelog: [from_over_into]: don't suggest From conversion when a generic From impl would cause a coherence conflict

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