[Parse/AST] Remove PoundDiagnosticDecl

There is no reson to make a AST node for '#error' and '#warning'
directives. Parser knows the diagnostics should be emitted or not.

test/SourceKit/SyntaxMapData/syntaxmap-pound-keyword.swift.response

@@ -178,5 +178,37 @@
       key.offset: 336,
       key.length: 6
     }
+  ],
+  key.diagnostics: [
+    {
+      key.line: 12,
+      key.column: 8,
+      key.filepath: syntaxmap-pound-keyword.swift,
+      key.severity: source.diagnostic.severity.error,
+      key.id: "pound_error",
+      key.description: "Error",
+      key.diagnostic_stage: source.diagnostic.stage.swift.parse,
+      key.ranges: [
+        {
+          key.offset: 224,
+          key.length: 7
+        }
+      ]
+    },
+    {
+      key.line: 14,
+      key.column: 10,
+      key.filepath: syntaxmap-pound-keyword.swift,
+      key.severity: source.diagnostic.severity.warning,
+      key.id: "pound_warning",
+      key.description: "Warning",
+      key.diagnostic_stage: source.diagnostic.stage.swift.parse,
+      key.ranges: [
+        {
+          key.offset: 255,
+          key.length: 9
+        }
+      ]
+    }
   ]
 }