PR17295: Do not allow explicit conversion functions to be used in cases where
an additional conversion (other than a qualification conversion) would be
required after the explicit conversion.

Conversely, do allow explicit conversion functions to be used when initializing
a temporary for a reference binding in direct-list-initialization.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191150 91177308-0d34-0410-b5e6-96231b3b80d8