Swift: make trap key prefixes readable

This replaces numeric tag-based prefixes with the actual tag name.
While this means in general slightly larger trap files, it aids
debugging them for a human.

In the future we can make this conditional on some kind of trap debug
option, but for the moment it does not seem detrimental.

swift/codegen/lib/cpp.py

@@ -83,7 +83,6 @@ class TagBase:
 class Tag:
     name: str
     bases: List[TagBase]
-    index: int
     id: str
 
     def __post_init__(self):