Currently clang looks up the coroutine_traits ClassTemplateDecl
everytime it looks up the promise type. This is unnecessary
as coroutine_traits doesn't change between promise type lookups.
This diff caches the coroutine_traits lookup.
I added log statements in the new lookupCoroutineTraits function
to ensure that LookupQualifiedName was only called once even
when multiple coroutines existed in the source file.