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/CommonNodes.py

@@ -12,6 +12,12 @@ COMMON_NODES = [
     Node('UnknownStmt', kind='Stmt'),
     Node('UnknownType', kind='Type'),
     Node('UnknownPattern', kind='Pattern'),
+    Node('Missing', kind='Syntax'),
+    Node('MissingDecl', kind='Decl'),
+    Node('MissingExpr', kind='Expr'),
+    Node('MissingStmt', kind='Stmt'),
+    Node('MissingType', kind='Type'),
+    Node('MissingPattern', kind='Pattern'),
 
     # code-block-item = (decl | stmt | expr) ';'?
     Node('CodeBlockItem', kind='Syntax', omit_when_empty=True,