Introduce a new syntax kind to represent missing base nodes

In the future, we only want to attach a source presence to tokens, not to nodes. All concreate nodes which are missing can be represented by creating the node and marking all tokens as missing, but if a syntax node carries a child, that has a base kind (like `Decl`), we can’t decide which concrete node to instantiate. Introduce `MissingDecl` etc. node for this purpose.

rdar://97908258
rdar://97775360

utils/gyb_syntax_support/NodeSerializationCodes.py

@@ -261,6 +261,12 @@ SYNTAX_NODE_SERIALIZATION_CODES = {
     'BackDeployAttributeSpecList' : 257,
     'BackDeployVersionList' : 258,
     'BackDeployVersionArgument' : 259,
+    'Missing': 260,
+    'MissingDecl': 261,
+    'MissingExpr': 262,
+    'MissingStmt': 263,
+    'MissingType': 264,
+    'MissingPattern': 265,
 }