[SE-0276] Implement multi-pattern catch clauses

Like switch cases, a catch clause may now include a comma-
separated list of patterns. The body will be executed if any
one of those patterns is matched.

This patch replaces `CatchStmt` with `CaseStmt` as the children
of `DoCatchStmt` in the AST. This necessitates a number of changes
throughout the compiler, including:
- Parser & libsyntax support for the new syntax and AST structure
- Typechecking of multi-pattern catches, including those which
  contain bindings.
- SILGen support
- Code completion updates
- Profiler updates
- Name lookup changes

utils/gyb_syntax_support/NodeSerializationCodes.py

@@ -244,6 +244,8 @@ SYNTAX_NODE_SERIALIZATION_CODES = {
     'PoundFilePathExpr': 240,
     'DerivativeRegistrationAttributeArguments': 241,
     'QualifiedDeclName': 242,
+    'CatchItem': 243,
+    'CatchItemList': 244,
 }