Add caching when looking up coroutine_traits

Summary:
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.

Patch by Tanoy Sinha!

Test Plan:
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.

Reviewers: modocache, GorNishanov

Reviewed By: modocache

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48981

llvm-svn: 337103