Refactor noexcept macros

Define noexcept using _MSC_VER instead of _MSC_FULL_VER (first release
of MSVC 2015 should have it), remove redundant PUGIXML_HAS_NOEXCEPT and
define PUGIXML_NOEXCEPT_IF_NOT_COMPACT in terms of PUGIXML_NOEXCEPT.