[AutoDiff] Fix unexpected non-differentiable property access error.

[dan-zheng]

Add base type parameter to TangentStoredPropertyRequest.

Use TypeBase::getTypeOfMember instead of VarDecl::getType to correctly
compute the member type of original stored properties, using the base type.

Resolves SR-13134.

---

Example:

import _Differentiation

struct Complex<T: FloatingPoint> {
  var real: T
  var imaginary: T
}

extension Complex: Differentiable where T: Differentiable {
  typealias TangentVector = Complex
}
extension Complex: AdditiveArithmetic {}

@differentiable
func addRealComponents(lhs: Complex<Float>, rhs: Complex<Float>) -> Float {
  return lhs.real + rhs.real
}

Before:

$ swiftc complex.swift
complex.swift:13:2: error: function is not differentiable
@differentiable
~^~~~~~~~~~~~~~
complex.swift:14:6: note: when differentiating this function definition
func addRealComponents(lhs: Complex<Float>, rhs: Complex<Float>) -> Float {
     ^
complex.swift:15:14: note: cannot differentiate access to property 'Complex.real' because property type 'T' does not conform to 'Differentiable'
  return lhs.real + rhs.real
             ^

After: no error, as expected.

[dan-zheng]

Note: adding a base type parameter to TangentStoredPropertyRequest results in fewer cache hits.
It may be possible to fix SR-13134 without adding a base type parameter, I may take a look later.

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

@swift-ci Please smoke test