[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.