[Sema] Corrected the warn-on-throw-from-noexcept behavior to include nothrow

Discovered that 'nothrow' (which is supposed to be an alias for noexcept)
was not warning with a throw inside of it. This patch corrects the behavior
previously created to add 'nothrow' to this list.

Differential Revision: https://reviews.llvm.org/D38203

llvm-svn: 314229