Fixes #59100

The above referenced issue was causing a compiler crash when writing
differentiable protocols and corresponding implementers, without importing
the `_Differentiation` module.

Changes in this CR fix the issue by marking any `@differentiable`
attributes as invalid, if the `_Differentiation` module has not been
imported. This ignores the `@differentiable` attributes when the
protocol witnesses are being verified. Witness verification was previously
leading to an error (due to missing `@differentiable` attribute on the protocol
requirement implementer), and the corresponding diagnostic emission code was
then leading to a crash, because it was expecting the `_Differentiation`
module to be present.

Author: jkshtj

lib/Sema/TypeCheckAttr.cpp

@@ -5789,7 +5789,8 @@ IndexSubset *DifferentiableAttributeTypeCheckRequest::evaluate(
 void AttributeChecker::visitDifferentiableAttr(DifferentiableAttr *attr) {
   // Call `getParameterIndices` to trigger
   // `DifferentiableAttributeTypeCheckRequest`.
-  (void)attr->getParameterIndices();
+  if (attr->getParameterIndices() == nullptr)
+    attr->setInvalid();
 }
 
 /// Type-checks the given `@derivative` attribute `attr` on declaration `D`.

test/AutoDiff/Sema/differentiable_swift_without_import.swift

@@ -0,0 +1,19 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {
+  @differentiable(reverse) // expected-error {{@differentiable attribute used without importing module '_Differentiation'}}
+  func req()
+}
+
+struct S: P {
+  // Had the `_Differentiation` module been imported, this conformance 
+  // requirement would have generated an error, complaining about the 
+  // missing `@derivative` attribute. However, because the `_Differentiation`
+  // module import is missing, the `@differential` attribute on `P` is marked
+  // invalid and therefore not used while validating witnesses (specifically `S.req()`) 
+  // for the `P.req()` requirement.
+  //
+  // **NOTE** - This test can likely be removed when/if the differentiation module is 
+  // available by default in the Swift stdlib.
+  public func req() {}
+}