[clang] Replace SmallSet with SmallPtrSet (NFC) (#154262)
This patch replaces SmallSet<T *, N> with SmallPtrSet<T *, N>. Note
that SmallSet.h "redirects" SmallSet to SmallPtrSet for pointer
element types:
template <typename PointeeType, unsigned N>
class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N>
{};
We only have 30 instances that rely on this "redirection", with about
half of them under clang/. Since the redirection doesn't improve
readability, this patch replaces SmallSet with SmallPtrSet for pointer
element types.
I'm planning to remove the redirection eventually.
