fix: Support generics in extract_function assist

This change attempts to resolve issue rust-lang#7637: Extract into Function does not
create a generic function with constraints when extracting generic code.

In `FunctionBody::analyze_container`, when the ancestor matches `ast::Fn`, we
can perserve both the `generic_param_list` and the `where_clause`. These can
then be included in the newly extracted function output via `format_function`.

From what I can tell, the only other ancestor type that could potentially have
a generic param list would be `ast::ClosureExpr`. In this case, we perserve the
`generic_param_list`, but no where clause is ever present.

In this initial implementation, only generic type parameters (and their
associated where clauses) which are referenced in the extracted function body
are copied to the new function signature. Const and Lifetime params are ignored
for now.

I don't believe this implementation will work in contexts where the generic
parameters and where clauses are defined multiple layers above the function
being extracted, such as with nested function declarations. Resolving this
seems like another obvious improvement, but one that will potentially require
more significant changes to the structure of `analyze_container` that I wasn't
comfortable trying to make as a first change.

Author: DorianListens

crates/hir/src/lib.rs

@@ -3285,6 +3285,15 @@ impl Type {
         let tys = hir_ty::replace_errors_with_variables(&(self.ty.clone(), to.ty.clone()));
         hir_ty::could_coerce(db, self.env.clone(), &tys)
     }
+
+    pub fn as_type_param(&self, db: &dyn HirDatabase) -> Option<TypeParam> {
+        match self.ty.kind(Interner) {
+            TyKind::Placeholder(p) => Some(TypeParam {
+                id: TypeParamId::from_unchecked(hir_ty::from_placeholder_idx(db, *p)),
+            }),
+            _ => None,
+        }
+    }
 }
 
 #[derive(Debug)]