As reported in https://bugs.llvm.org/show_bug.cgi?id=33235,
a noexcept function was unable to inherit from a nothrow defaulted
constructor. Attribute "nothrow" is supposed to be semantically
identical to noexcept, and in fact, a number of other places in the
code treat them identically.
This patch simply checks the RecordDecl for the correct attribute in
the case where no other exception specifier was set.