[Swift] Add initial support for Swift demangling

[bdash]

This adds:

1. A new plug-in for Swift-specific analysis.
2. Basic demangling, where symbols have a demangled name assigned to them.
3. Support for applying parameter and return types to functions during demangling. This is behind a disabled by default setting at present, as we are currently not able to represent parameters that are small structs being passed across multiple registers. As a result, applying type information can be more confusing than not having it at all.
4. Swift calling conventions for arm64. The Swift ABI repurposes three callee-saved registers for implicit
   parameters (self, error, and async context). Supporting the various combinations of these requires registering several different calling conventions. The demangler is taught to explicitly apply these calling conventions to functions that need them.

To be done in the future:

1. Apply parameter and return types to thunks, metadata accessors, and all of the other nonsense Swift supports.
2. Swift calling conventions for x86_64.

Fixes #7211 and initial work towards #3914.