[macros] Further improve specializing generics in macros

### Description

In https://github.com/apple/swift-syntax/pull/2450 we introduced

```
@freestanding(expression)
public macro checkGeneric_root() = #externalMacro(module: "MacroDefinition", type: "GenericToVoidMacro")

@freestanding(expression)
public macro checkGeneric() = #checkGeneric_root()
```

such that the DAS gets replaced in every expansion:

```
checkGeneric -> checkGeneric_root
```

---

but the following cases are not replaced yet:

Would it make sense to specialize macros with types that aren’t part of the second-level macro? Ie. would the following make sense?

```
macro gen(a: T) = #externalMacro …
macro genString(a: String) = #gen(a: a)
```

And then #genString(a: "x") would have the intermediate expansion step #gen(a: "x")

-------

Similarly, would it make sense to allow generic nesting in a second-level macro definition? Ie. would something like the following make sense?

```
macro gen(a: T) = #externalMacro …
macro genArray(a: [T]) = #gen>(a: a)
```

And then #genArray(a: [1]) would have the intermediate expansion step #gen>(a: [1])

----

https://github.com/apple/swift-syntax/pull/2450#discussion_r1479217425

### Steps to Reproduce

_No response_

#### Radar

rdar://122364628