[CSSolver] Handle situations when key path expression has an existential type

A way to mark key path as Sendable is to extend its type with `& Sendable`.
This is something that makes the type of a key path expression an existential
with superclass bound expressed as a known key path type.

Author: xedin

lib/Sema/CSApply.cpp

@@ -5009,6 +5009,12 @@ namespace {
         baseTy = fnTy->getParams()[0].getParameterType();
         leafTy = fnTy->getResult();
         isFunctionType = true;
+      } else if (auto *existential = exprType->getAs<ExistentialType>()) {
+        auto constrainedTy = existential->getConstraintType();
+        auto layout = constrainedTy->getExistentialLayout();
+        auto keyPathTy = layout.explicitSuperclass->castTo<BoundGenericType>();
+        baseTy = keyPathTy->getGenericArgs()[0];
+        leafTy = keyPathTy->getGenericArgs()[1];
       } else {
         auto keyPathTy = exprType->castTo<BoundGenericType>();
         baseTy = keyPathTy->getGenericArgs()[0];

lib/Sema/CSSimplify.cpp

@@ -12195,6 +12195,12 @@ ConstraintSystem::simplifyKeyPathConstraint(
     if (contextualTy->isPlaceholder())
       return true;
 
+    // Situations like `any KeyPath<...> & Sendable`.
+    if (contextualTy->isExistentialType()) {
+      contextualTy = contextualTy->getExistentialLayout().explicitSuperclass;
+      assert(contextualTy);
+    }
+
     // If there are no other options the solver might end up picking
     // `AnyKeyPath` or `PartialKeyPath` based on a contextual conversion.
     // This is an error during normal type-checking but okay in