c++: error with bit-fields and scoped enums [PR111895]

Here we issue a bogus error: invalid operands of types 'unsigned char:2'
and 'int' to binary 'operator!=' when casting a bit-field of scoped enum
type to bool.

In build_static_cast_1, perform_direct_initialization_if_possible returns
NULL_TREE, because the invented declaration T t(e) fails, which is
correct.  So we go down to ocp_convert, which has code to deal with this
case:
          /* We can't implicitly convert a scoped enum to bool, so convert
             to the underlying type first.  */
          if (SCOPED_ENUM_P (intype) && (convtype & CONV_STATIC))
            e = build_nop (ENUM_UNDERLYING_TYPE (intype), e);
but the SCOPED_ENUM_P is false since intype is <unnamed-unsigned:2>.
This could be fixed by using unlowered_expr_type.  But then
c_common_truthvalue_conversion/CASE_CONVERT has a similar problem, and
unlowered_expr_type is a C++-only function.

Rather than adding a dummy unlowered_expr_type to C, I think we should
follow [expr.static.cast]p3: "the lvalue-to-rvalue conversion is applied
to the bit-field and the resulting prvalue is used as the operand of the
static_cast."  There are no prvalue bit-fields, so the l-to-r conversion
performed in decay_conversion will get us an expression whose type is the
enum.

	PR c++/111895

gcc/cp/ChangeLog:

	* typeck.cc (build_static_cast_1): Call decay_conversion.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/scoped_enum12.C: New test.