[Sema] Correct nothrow inherited by noexcept

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.

Differential Revision: https://reviews.llvm.org/D38209

llvm-svn: 314462