SwiftCompilerSources: refactor DiagnosticEngine

* move it from the SIL to the AST module (where it belongs)
* change the signature of `diagnose` from `diagnose(location, .some_error)` to `diagnose(.some_error, at: location)`
* add an overload to allow passing a `SIL.Location` directly to `diagnose`
* add a `Diagnostic : Error` utility struct which allows throwing a `Diagnostic`

SwiftCompilerSources/Sources/SIL/DeclRef.swift

@@ -31,3 +31,9 @@ public struct DeclRef: CustomStringConvertible, NoReflectionChildren {
     lhs.bridged.isEqualTo(rhs.bridged)
   }
 }
+
+extension DeclRef: DiagnosticArgument {
+  public func _withBridgedDiagnosticArgument(_ fn: (BridgedDiagnosticArgument) -> Void) {
+    fn(bridged.asDiagnosticArgument())
+  }
+}