Swift: make trap key prefixes readable

[redsun82]

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.

[redsun82]

For reference, here's how a DB inconsistency from an integration test looks with this change:

[INVALID_KEY_SET] predicate iterable_decl_context_members(@iterable_decl_context id, int index, @decl member): The key set {id, index} does not functionally determine all fields.
Here is a pair of tuples that agree on the key set but differ at index 2:
Tuple 1 in row 4525: (77649,2,77699)
Tuple 2 in row 4526: (77649,2,230376)
	Relevant element: Tuple 1: id=77649
		Full ID for 77649: @"StructDecl_So10HeapObjectV"
	Relevant element: Tuple 2: id=77649
		Full ID for 77649: @"StructDecl_So10HeapObjectV"

The StructDecl instead of a number is the difference.

[AlexDenisov]

Love it!