Fix nothrow trait with multiple default constructors

Check all default ctors, not just the first one we see. This brings
__has_nothrow_constructor() in line with the other unary type traits.

A C++ class can have multiple default constructors but clang was only checking
the first one written, presumably due to ambiguity in the GNU specification.

MSVC has the same bug, while g++ has the correct implementation which we now
match.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199618 91177308-0d34-0410-b5e6-96231b3b80d8
MSVC 2013 type trait support

Implement type trait primitives used in the latest edition of the Microsoft
standard C++ library type_traits header.

With this change we can parse much of the Visual Studio 2013 standard headers,
particularly anything that includes <type_traits>.

Fully implemented, available in all language modes:

 * __is_constructible()
 * __is_nothrow_constructible()
 * __is_nothrow_assignable()

Partially implemented, semantic analysis WIP, available as MS extensions:

 * __is_destructible()
 * __is_nothrow_destructible()

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199619 91177308-0d34-0410-b5e6-96231b3b80d8