Explicit casts of S -> std::complex<T>

When calling `internal::cast<S, std::complex<T>>(x)`, clang often
generates an implicit conversion warning due to an implicit cast
from type `S` to `T`.  This currently affects the following tests:
- `basicstuff`
- `bfloat16_float`
- `cxx11_tensor_casts`

The implicit cast leads to widening/narrowing float conversions.
Widening warnings only seem to be generated by clang (`-Wdouble-promotion`).

To eliminate the warning, we explicitly cast the real-component first
from `S` to `T`.  We also adjust tests to use `internal::cast` instead
of `static_cast` when a complex type may be involved.