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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314229 91177308-0d34-0410-b5e6-96231b3b80d8