SIL: Use the right generic signature for computing SIL function type for coroutine accessors

The function type's generic signature will have already been dropped
at this point if all generic parameters were fully concrete, so use
the original generic signature to compute the reduced type of the
yield type.

Fixes #61040.

Author: slavapestov

lib/SIL/IR/SILFunctionType.cpp

@@ -1960,9 +1960,12 @@ static CanSILFunctionType getSILFunctionType(
 
     if (reqtSubs) {
       valueType = valueType.subst(*reqtSubs);
+      coroutineSubstYieldType = valueType->getReducedType(
+          genericSig);
+    } else {
+      coroutineSubstYieldType = valueType->getReducedType(
+          accessor->getGenericSignature());
     }
-
-    coroutineSubstYieldType = valueType->getReducedType(genericSig);
   }
 
   bool shouldBuildSubstFunctionType = [&]{

test/SILGen/constrained_extensions.swift

@@ -227,3 +227,16 @@ func referenceNestedTypes() {
   _ = Array<AnyObject>.NestedClass()
   _ = Array<AnyObject>.DerivedClass()
 }
+
+struct S<T> {
+  struct X {}
+}
+
+// CHECK-LABEL: sil hidden [ossa] @$s22constrained_extensions1SVAASiRszlEyAC1XVySi_GSicir : $@yield_once @convention(method) (Int, S<Int>) -> @yields S<Int>.X
+extension S<Int> {
+  subscript(index:Int) -> X {
+    _read {
+      fatalError()
+    }
+  }
+}