swift234

Do not suppress optional coercion in operators with a nil-literal operand.

This is the hack that has been used to reject things like:
  var i: Int = ...
  if i == nil { }
in the past.

The hack is inconsistent with normal treatment of mixed optional &
non-optional operands, and will be replaced with a warning instead of
treating it as a failure to type check.

There is still a case that we still fail type checking on -
Unsafe*Pointer<> compares to nil. That will be addressed by a separate
commit.

The new warning will be addressed by rdar://problem/27457457. When the
new warnings are updated the test cases modified here will again need to
be updated based on the text of the new warning.