As discussed in the mail thread https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/T64_dW3WKUk "Calling noexcept function throug non-noexcept pointer is undefined behavior?", such a call should not be UB. However, Clang currently warns about it.
There is no cheap check whether two function type_infos only differ in noexcept, so pass those two type_infos as additional data to the function_type_mismatch handler (with the optimization of passing a null "static callee type" info when that is already noexcept, so the additional check can be avoided anyway). For the Itanium ABI (which appears to only record noexcept information for pointer-to-function type_infos, not for function type_infos themselves), we then need to check the mangled names for occurrence of "Do" representing "noexcept".