MacroSystem expansion detached nodes lose diagnostic context

### Description

Macro expansions create diagnostics that are not relative to the given context (because they're `detached`), which means that there's no way to collocate diagnostics with the original source.

### Steps to Reproduce

You can take the `AddBlocker` macro demo from [swift-macro-examples](/DougGregor/swift-macro-examples) and expand the given macro:

```swift
let x = 1
let y = 2
let z = 3
#addBlocker(x * y + z)
```

If you expand as is, the context's diagnostic for the "+" will be relative to the `#addBlocker` expression macro node, not the entire source (starting at `let x = 1`), so formatting the diagnostics against the original source will render the diagnostic in the wrong place:

```swift
let x = 1
let y = 2
// ╰─ warning: blocked an add; did you mean to subtract?
let z = 3
#addBlocker(x * y + z)
```

It can be rendered correctly by _not_ detaching the context during expansion [here](https://github.com/apple/swift-syntax/blob/f6953015ba2d8c97df61760b614c59c084b72c7d/Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift#L83):

```diff
-node: node.detach(in: context),
+node: node, //.detach(in: context),
```
```swift
let x = 1
let y = 2
let z = 3
#addBlocker(x * y + z)
// ╰─ warning: blocked an add; did you mean to subtract?
```

Diagnostics added during expansion should ideally be adjusted from where they were detached.