[AST/Sema] Add a diagnostic group ExplicitSendable to replace -require-explicit-sendable

Always run explicit `Sendable` checks on public types and suppress
warning printing by default instead of using a special compiler argument.

Resolves: rdar://162394810

userdocs/diagnostics/explicit-sendable-annotations.md

@@ -0,0 +1,30 @@
+# Explicit Sendable annotations on public type declarations
+
+If a public type doesn't have an explicit Sendable or non-Sendable annotation it is sometimes hard to discern whether that is intentional or not, especially if a type could be Sendable.
+
+## Overview
+
+The Swift compiler would emit a warning if a public type has none of the following:
+
+  - A conformance to `Sendable` protocol;
+  - An unavailable conformance to `Sendable` protocol;
+  - `~Sendable` conformance to suppress the inference.
+
+Let's consider a simple public type without any Senable annotations:
+
+```
+public struct S {
+    let x: Int
+}
+```
+
+When compiling with `-Wwarning ExplicitSendable` the following warning is going to be produced by the Swift compiler:
+
+```
+1 | public struct S {
+  |               |- warning: public struct 'S' does not specify whether it is 'Sendable' or not [#ExplicitSendable]
+  |               |- note: consider making struct 'S' conform to the 'Sendable' protocol
+  |               `- note: make struct 'S' explicitly non-Sendable to suppress this warning
+2 |     let x: Int
+3 | }
+```