ThreadSafeBox is a `class` wrapping a `Mutex<Value>`. When stored as
a property of another class or actor, the outer reference type already
provides identity, so the extra ThreadSafeBox class indirection is just a
redundant heap allocation. Replace `ThreadSafeBox<T>` with `Mutex<T>` at
those sites.
Function-local ThreadSafeBox uses are unchanged since they need class
identity to be capturable in `@Sendable` closures.
CustomBuildServerTestProject.buildServerBox is also unchanged: it is
value-captured into a closure stored before `super.init`, which a
~Copyable Mutex can't support without a class wrapper.
Port SourceKitD's fileprivate `computeIfNil` extension from ThreadSafeBox
to Mutex (now `borrowing`).
Delete sourcekit-lsp's `Sources/SwiftExtensions/ThreadSafeBox.swift`
and the now-unused `NSLock+WithLock.swift`.
Use the `Mutex`-backed `ThreadSafeBox` from swift-tools-protocols'
Adjust call sites for the new API: e.g.
`foo.value.mutate()` with `foo.withLock { $0.mutate() }` because it
doesn't provide `_modify` accessor.
Rintaro Ishizaki
Bhushan-Aditya
Anthony Latsis
Owen Voorhees
Alex Hoppen