[NFC] [Frontend] Correct the use of 'auto' in SemaCoroutine and CGCoroutine

We should only use 'auto' in case we can know the type from the right
hand side of the expression. Also we need keep '*' around if the type is
a pointer actually. Few uses of 'auto' in SemaCoroutine.cpp and
CGCoroutine.cpp violates the rule. This commit tries to fix it.