[AutoDiff] Enable @derivative attribute qualified declaration names.

[dan-zheng]

Enable qualified declaration names in @derivative attribute, just like
@transpose attribute.

DerivativeAttr now stores a base type TypeRepr *, which is non-null for
parsed attributes that reference a qualified original declaration.

Add TypeResolutionFlags::AllowModule flag to enable module lookup via
TypeChecker::lookupMember given a ModuleType.

Add tests for type-qualified and module-qualified declaration names.

Resolves TF-1058.

---

Examples:

func foo(_ x: Float) -> Float { x }

// 1. Module qualification.
// Immediately useful for derivative registration use cases: TF-1002.
// May be replaced by proper module qualification/`DeclNameRef` work:
// - https://forums.swift.org/t/pitch-fully-qualified-name-syntax/28482
// - https://forums.swift.org/t/what-the-eff-is-declnameref/31594
@derivative(of: MyModule.foo)
func vjpFoo(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
  (foo(x), { $0 })
}

struct Struct {
  func instanceMethod(_ x: Float) -> Float { x }
}
extension Struct {
  // 2. Type qualification.
  // Not very useful for `@derivative` attribute, because the original
  // declaration type context can always be inferred.
  // Currently useful for `@transpose` attribute until TF-1060.
  @derivative(of: Struct.instanceMethod)
  func vjpInstanceMethod(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
    (instanceMethod(x), { $0 })
  }
}

[rxwei]

Thank you!

[dan-zheng]

@brentdax: I added you as a reviewer because this ad-hoc support for module-qualified and type-qualified declaration names for @derivative and @transpose attributes overlaps with your work on module-qualified names.

Some context: the main "qualified name" use case for @derivative and @transpose is module-qualified names. Type-qualified names are no longer necessary after a @transpose type-checking rules revamp (TF-1060).

After syntax exists for your work on module-qualified names (e.g. Module::fooName), we can drop our ad-hoc support and switch to your work!

[dan-zheng]

@swift-ci Please test

[dan-zheng]

Merging to unblock progress. I'll cherry-pick this change to tensorflow branch.
Happy to address any review feedback later!