[ConstraintSystem] Infer some of the bindings attributes on demand

[xedin]

One step closer to being able to make potential binding inference incremental.

Let's make most of the flags in PotentialBindings be computed on demand based
on information captured in the bindings, including:

• IsHole - could be determined based on a presence of a single binding to a hole type;
• NumDefaultableBindings - just a number of bindings originated in defaultable constraint;
• IsPotentiallyIncomplete - determined based on bindings and locator;
• SubtypeOfExistentialType - determined based on recorded bindings;

Most of the flags here are only used for ranking and recorded in a BindingScore
so don't have to be re-computed multiple times.

[xedin]

@swift-ci please test

[xedin]

@swift-ci please test source compatibility

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - afa1198

[xedin]

@swift-ci please test Linux platform

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - afa1198

[xedin]

@nkcsgexi @artemcm Linux CI is failing with:

/home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/swift-driver/Tests/SwiftDriverTests/IncrementalCompilationTests.swift:15:18: error: module 'SwiftDriver' was not compiled for testing
19:20:48 @testable import SwiftDriver
19:20:48 ~~~~~~~~~~       ^

[artemcm]

@nkcsgexi @artemcm Linux CI is failing with:

/home/buildnode/jenkins/workspace/swift-PR-Linux/branch-main/swift-driver/Tests/SwiftDriverTests/IncrementalCompilationTests.swift:15:18: error: module 'SwiftDriver' was not compiled for testing
19:20:48 @testable import SwiftDriver
19:20:48 ~~~~~~~~~~       ^

Investigating! Thanks.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - afa1198

[xedin]

@swift-ci please test

[hborla]

Nice!