[AutoDiff] Revamp derivative type calculation using llvm::Expected.

[dan-zheng]

Create DerivativeFunctionTypeError representing all potential derivative
function type calculation errors.

Make AnyFunctionType::getAutoDiffDerivativeFunctionLinearMapType return
llvm::Expected<AnyFunctionType *>. This is much safer and caller-friendly
than performing assertions.

Delete hacks in @differentiable and @derivative attribute type-checking
for verifying that Differentiable.TangentVector type witnesses are valid:
this is no longer necessary.

Robust fix for TF-521: invalid Differentiable conformances during
@derivative attribute type-checking.

Resolves SR-12793: bad interaction between @differentiable and @derivative
attribute type-checking and Differentiable derived conformances.

Todos

• Requestify AnyFunctionType::getAutoDiffDerivativeFunctionType and AnyFunctionType::getAutoDiffDerivativeFunctionLinearMapType to cache results.

Example

import _Differentiation

struct Struct: Differentiable {
  var x: Float

  @differentiable
  func method() -> Float { x }

  mutating func move(along direction: TangentVector) {}
}

Before (the bad interaction):

$ swiftc test.swift
test.swift:9:39: error: reference to invalid associated type 'TangentVector' of type 'Struct'
  mutating func move(along direction: TangentVector) {}
                                      ^

After: no error.

[dan-zheng]

@CodaFi: the removal of this code should address your concerns from https://github.com/apple/swift/pull/30936/files#r406541238:

This mix of semantic and syntactico-semantic checks doesn't feel right. Is the idea that you're trying to check for a conformance to this protocol, or to a well-formed conformance of this protocol?

We directly use TypeChecker::conformsToProtocol now, which is syntactic. Semantic checks (requesting TangentVector type witnesses) are performed and diagnosed on-demand.

[dan-zheng]

@swift-ci Please test

[dan-zheng]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - cb2a709f55de1cc984bcf648cb61d6aab9ca5ada

[dan-zheng]

Merging now to unblock progress, I'm happy to address any feedback later!
I plan to address #31727 (comment) in a follow-up.