Fix __libcpp_is_constructible for source types with explicit conversion operators.

Previously __libcpp_is_constructible checked the validity of reference
construction using 'eat<To>(declval<From>())' but this doesn't consider
From's explicit conversion operators. This patch teaches __libcpp_is_constructible
how to handle these cases. To do this we need to check the validity
using 'static_cast<To>(declval<From>())'. Unfortunately static_cast allows
additional base-to-derived and lvalue-to-rvalue conversions, which have to be
checked for and manually rejected.

While implementing these changes I discovered that Clang incorrectly
rejects `static_cast<int&&>(declval<float&>())` even though
`int &&X(declval<float&>())` is well formed. In order to tolerate this bug
the `__eat<T>(...)` needs to be left in-place. Otherwise it could be replaced
entirely with the new static_cast implementation.

Thanks to Walter Brown for providing the test cases.

llvm-svn: 285786