Don't perform ADL when looking up operator=; there is no non-member form of

Don't perform ADL when looking up operator=; there is no non-member form of
that function, and apart from being slow, this is unnecessary: ADL can trigger
instantiations that are not permitted here. The standard isn't *completely*
clear here, but this seems like the intent, and in any case this approach is
permitted by [temp.inst]p7.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218330 91177308-0d34-0410-b5e6-96231b3b80d8