CodeGen: Base the conditional cleanup machinery on variadic templates

CodeGen: Base the conditional cleanup machinery on variadic templates

This is complicated by the fact that we can't simply use side-effecting
calls in an argument list without losing all guarantees about the order
they're emitted. To keep things deterministic we use tuples and brace
initialization, which thankfully guarantees evaluation order.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232121 91177308-0d34-0410-b5e6-96231b3b80d8