New force_warn diagnostic builder and ensure cap-lints doesn't reduce force_warn level

src/test/ui/lint/force-warn/force-warn-cap-lints-warn.rs

@@ -0,0 +1,15 @@
+// compile-flags: --cap-lints warn  --force-warns rust-2021-compatibility -Zunstable-options
+// check-pass
+#![allow(ellipsis_inclusive_range_patterns)]
+
+pub fn f() -> bool {
+    let x = 123;
+    match x {
+        0...100 => true,
+        //~^ WARN range patterns are deprecated
+        //~| WARN this is accepted in the current edition
+        _ => false,
+    }
+}
+
+fn main() {}

src/test/ui/lint/force-warn/force-warn-cap-lints-warn.stderr

@@ -0,0 +1,12 @@
+warning: `...` range patterns are deprecated
+  --> $DIR/force-warn-cap-lints-warn.rs:8:10
+   |
+LL |         0...100 => true,
+   |          ^^^ help: use `..=` for an inclusive range
+   |
+   = note: `--force-warns ellipsis-inclusive-range-patterns` implied by `--force-warns rust-2021-compatibility`
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see issue #80165 <https://github.com/rust-lang/rust/issues/80165>
+
+warning: 1 warning emitted
+

src/test/ui/lint/force-warn/force-warns-cap-lints-allow.stderr

@@ -1,5 +1,5 @@
 warning: trait objects without an explicit `dyn` are deprecated
-  --> $DIR/force-warns-cap-lints.rs:6:25
+  --> $DIR/force-warns-cap-lints-allow.rs:6:25
    |
 LL | pub fn function(_x: Box<SomeTrait>) {}
    |                         ^^^^^^^^^ help: use `dyn`: `dyn SomeTrait`