Preserve exception specs in function decl merging.

Exception specs are not part of the canonical type, but we shouldn't
drop them just because we merged a noreturn attribute.

Fixes PR17110.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190206 91177308-0d34-0410-b5e6-96231b3b80d8