[CodeGen] Emit a call instruction instead of an invoke if the called

llvm function is marked nounwind

This fixes cases where an invoke is emitted, despite the called llvm
function being marked nounwind, because ConstructAttributeList failed to
add the attribute to the attribute list. llvm optimization passes turn
invokes into calls and optimize away the exception handling code, but
it's better to avoid emitting the code in the front-end if the called
function is known not to raise an exception.

Differential Revision: https://reviews.llvm.org/D83906