Swift: Salvage

[geoffw0]

Salvage some commits from old branches that have become dead ends:

• add BuiltinLiteralExpr.getValueString(); this is a unified / consistent interface for accessing the value of literals. The specific use I had for it is gone, but I can imagine we will want this in future as doing this case-by-case is fiddly, ugly and brittle.
• new qldoc comments around literals.
• new test cases for data flow through dictionaries.

TODO:

• [ ] BuiltinLiteralExpr.getValueString() should have tests.