Add support for _specialize(exported: true, ...)

This attribute allows to define a pre-specialized entry point of a
generic function in a library.

The following definition provides a pre-specialized entry point for
`genericFunc(_:)` for the parameter type `Int` that clients of the
library can call.

```
@_specialize(exported: true, where T == Int)
public func genericFunc<T>(_ t: T) { ... }
```

Pre-specializations of internal `@inlinable` functions are allowed.

```
@usableFromInline
internal struct GenericThing<T> {
  @_specialize(exported: true, where T == Int)
  @inlinable
  internal func genericMethod(_ t: T) {
  }
}
```

There is syntax to pre-specialize a method from a different module.

```
import ModuleDefiningGenericFunc

@_specialize(exported: true, target: genericFunc(_:), where T == Double)
func prespecialize_genericFunc(_ t: T) { fatalError("dont call") }

```

Specially marked extensions allow for pre-specialization of internal
methods accross module boundries (respecting `@inlinable` and
`@usableFromInline`).

```
import ModuleDefiningGenericThing
public struct Something {}

@_specializeExtension
extension GenericThing {
  @_specialize(exported: true, target: genericMethod(_:), where T == Something)
  func prespecialize_genericMethod(_ t: T) { fatalError("dont call") }
}
```

rdar://64993425

Author: aschwaighofer

docs/ABI/Mangling.rst

@@ -995,6 +995,7 @@ Function Specializations
 ::
 
   specialization ::= type '_' type* 'Tg' SPEC-INFO     // Generic re-abstracted specialization
+  specialization ::= type '_' type* 'Ts' SPEC-INFO     // Generic re-abstracted prespecialization
   specialization ::= type '_' type* 'TG' SPEC-INFO     // Generic not re-abstracted specialization
   specialization ::= type '_' type* 'Ti' SPEC-INFO     // Inlined function with generic substitutions.
 

include/swift/AST/Attr.def

@@ -584,6 +584,11 @@ SIMPLE_DECL_ATTR(globalActor, GlobalActor,
   APIStableToAdd | APIBreakingToRemove,
   104)
 
+SIMPLE_DECL_ATTR(_specializeExtension, SpecializeExtension,
+  OnExtension | UserInaccessible |
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  105)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS