[CSApply] Allow @dynamicMemberLookup subscripts to take sendable key path

The changes to support `& Sendable` composition with key path
types adjusted `buildKeyPathDynamicMemberArgExpr` to support
that but the use-site erroneously still cast index type or
`BoundGenericType` as before.

Resolves: #75244
Resolves: rdar://131768785

Author: xedin

lib/Sema/CSApply.cpp

@@ -5409,8 +5409,8 @@ namespace {
         auto indexType = getTypeOfDynamicMemberIndex(overload);
         Expr *argExpr = nullptr;
         if (overload.choice.isKeyPathDynamicMemberLookup()) {
-          argExpr = buildKeyPathDynamicMemberArgExpr(
-              indexType->castTo<BoundGenericType>(), componentLoc, memberLoc);
+          argExpr = buildKeyPathDynamicMemberArgExpr(indexType, componentLoc,
+                                                     memberLoc);
         } else {
           auto fieldName = overload.choice.getName().getBaseIdentifier().str();
           argExpr = buildDynamicMemberLookupArgExpr(fieldName, componentLoc,

test/attr/attr_dynamic_member_lookup.swift

@@ -833,3 +833,10 @@ public extension S3_54864 {
     get { s2_54864_instance[keyPath: member] } // expected-error {{key path with root type 'S3_54864' cannot be applied to a base of type 'S2_54864'}}
   }
 }
+
+// https://github.com/swiftlang/swift/issues/75244
+struct WithSendable {
+  subscript(dynamicMember member: KeyPath<String, Int> & Sendable) -> Bool { // Ok
+    get { false }
+  }
+}