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

lib/SILOptimizer/Transforms/GenericSpecializer.cpp

@@ -81,7 +81,7 @@ bool GenericSpecializer::specializeAppliesInFunction(SILFunction &F) {
       auto *Callee = Apply.getReferencedFunctionOrNull();
       if (!Callee)
         continue;
-      if (!Callee->isDefinition()) {
+      if (!Callee->isDefinition() && !Callee->hasPrespecialization()) {
         ORE.emit([&]() {
           using namespace OptRemark;
           return RemarkMissed("NoDef", I)