Fix tuple's conditionally explicit constructors for very weird user
types.

It seems some people like to write types that can explicitly convert
to anything, but cannot be used to explicitly construct anything.

This patch makes tuple tolerate such types, as is required
by the standard.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365074 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed