[ThinLTO] Use DenseSet instead of SmallPtrSet for holding GUIDs

Should fix some more bot failures from r291108.
This should have been a DenseSet, since GUID is not a pointer type.
It caused some bots to fail, but for some reason I wasnt't getting a
build failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291115 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed