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