[libc++][NFC] Fix unparenthesized comma expression in mem-initializer (#89605)

#84050 resolves class member access expressions naming members of the
current instantiation prior to instantiation. In testing, it has
revealed a mem-initializer in the move constructor of
`invocable_with_telemetry` that uses an unparenthesized comma expression
to initialize a non-static data member of pointer type. This patch fixes it.

Author: sdkrystian

libcxx/test/support/invocable_with_telemetry.h

@@ -31,7 +31,7 @@ class invocable_with_telemetry {
   constexpr invocable_with_telemetry(invocable_with_telemetry&& other)
     requires std::move_constructible<F>
       : f_(std::move(other.f_)),
-        telemetry_(assert(other.telemetry_ != nullptr), std::exchange(other.telemetry_, nullptr)) {
+        telemetry_((assert(other.telemetry_ != nullptr), std::exchange(other.telemetry_, nullptr))) {
     ++telemetry_->moves;
   }