[AutoDiff upstream] Add derivative function SILDeclRefs.

[dan-zheng]

@differentiable attribute on protocol requirements and non-final class members
will produce derivative function entries in witness tables and vtables.

This patch adds an optional derivative function configuration
(AutoDiffDerivativeFunctionIdentifier) to SILDeclRef to represent these
derivative function entries.

Derivative function configurations consist of:

• A derivative function kind (JVP or VJP).
• Differentiability parameter indices.

sil-decl-ref ::= '#' sil-identifier ('.' sil-identifier)* sil-decl-subref?
sil-decl-subref ::= '!' sil-decl-subref-part ('.' sil-decl-lang)? ('.' sil-decl-autodiff)?
sil-decl-autodiff ::= sil-decl-autodiff-kind '.' sil-decl-autodiff-indices
ail-decl-autodiff-kind ::= 'jvp' | 'vjp'
sil-decl-autodiff-indices ::= [SU]+
                               ^~ 'S' is set, 'U' is unset

Resolves TF-1209.
Enables TF-1212: upstream derivative function entries in witness tables/vtables.

---

Derivative function SILDeclRef examples:

protocol Protocol {
  @differentiable(wrt: (x, y))
  func f(_ x: Float, _ y: Float) -> Float
}

sil hidden @witness_method : $@convention(thin) <T where T : Protocol> (@in T) -> () {
bb0(%0 : $*T):
  // Original function reference.
  %1 = witness_method $T, #Protocol.f : <Self where Self : Protocol> (Self) -> (Float, Float) -> Float : $@convention(witness_method: Protocol) <τ_0_0 where τ_0_0 : Protocol> (@in_guaranteed τ_0_0) -> (Float, Float) -> Float

  // Derivative function references.
  %2 = witness_method $T, #Protocol.f!jvp.SSS : <Self where Self : Protocol> (Self) -> (Float, Float) -> Float : $@convention(witness_method: Protocol) <τ_0_0 where τ_0_0 : Protocol> (@in_guaranteed τ_0_0) -> (Float, Float) -> Float
  %3 = witness_method $T, #Protocol.f!jvp.UUS : <Self where Self : Protocol> (Self) -> (Float, Float) -> Float : $@convention(witness_method: Protocol) <τ_0_0 where τ_0_0 : Protocol> (@in_guaranteed τ_0_0) -> (Float, Float) -> Float
  %4 = witness_method $T, #Protocol.f!vjp.SSS : <Self where Self : Protocol> (Self) -> (Float, Float) -> Float : $@convention(witness_method: Protocol) <τ_0_0 where τ_0_0 : Protocol> (@in_guaranteed τ_0_0) -> (Float, Float) -> Float
  %5 = witness_method $T, #Protocol.f!vjp.UUS : <Self where Self : Protocol> (Self) -> (Float, Float) -> Float : $@convention(witness_method: Protocol) <τ_0_0 where τ_0_0 : Protocol> (@in_guaranteed τ_0_0) -> (Float, Float) -> Float

  %6 = tuple ()
  return %6 : $()
}

[dan-zheng]

@slavapestov: would you like to review this patch, as a SIL code owner?
You previously approved this patch in 2018 on tensorflow branch: #21224

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

This can be simplified after #29105:

Suggested change

	CanGenericSignature derivativeCanGenSig;
	if (derivativeGenericSignature)
	derivativeCanGenSig = derivativeGenericSignature->getCanonicalSignature();
	auto derivativeCanGenSig = derivativeGenericSignature.getCanonicalSignature();

I'll land this in a follow-up.

[dan-zheng]

@swift-ci Please smoke test macOS platform

[dan-zheng]

Both macOS and Linux CI failed mysteriously. Retriggering.
@swift-ci Please smoke test Linux

[dan-zheng]

@swift-ci Please smoke test Linux

[dan-zheng]

@swift-ci Please test

[dan-zheng]

SwiftPM tests on Linux seem to have flaked again (for the fourth time):

:0: error: module file '/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/tmp_swiftpm/ModuleCache/34DIH411VVHZV/CDispatch-E28X8IBKETOX.pcm' is out of date and needs to be rebuilt: signature mismatch
10:31:25 :0: note: imported by module 'CoreFoundation' in '/home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/tmp_swiftpm/ModuleCache/34DIH411VVHZV/CoreFoundation-1HECCF3FGVFVS.pcm'
10:31:25 :0: error: missing required modules: 'CDispatch', '_SwiftDispatchOverlayShims'
10:31:25 /home/buildnode/jenkins/workspace/swift-PR-Linux-smoke-test/branch-master/swiftpm/Tests/PackageLoadingTests/PDLoadingTests.swift:50: error: PackageDescription4_2LoadingTests.testBasics : failed - 
10:31:25 Test Case 'PackageDescription4_2LoadingTests.testBasics' failed (0.778 seconds)
10:31:25 Test Suite 'PackageDescription4_2LoadingTests' failed at 2020-03-22 17:31:06.456
10:31:25 	 Executed 1 test, with 1 failure (0 unexpected) in 0.778 (0.778) seconds
10:31:25 Test Suite 'Selected tests' failed at 2020-03-22 17:31:06.456
10:31:25 	 Executed 1 test, with 1 failure (0 unexpected) in 0.778 (0.778) seconds

@swift-ci Please smoke test Linux

[dan-zheng]

Merging to unblock progress. Happy to address feedback later!