[clang-repl] Better match the underlying architecture.

In cases where -fno-integrated-as is specified we should overwrite the
EmitAssembly action as well.

We also should rely on the target triple from the process at least until we
implement out-of-process execution.

This patch should improve clang-repl on AIX.

Discussion available at: https://reviews.llvm.org/D96033

Differential revision: https://reviews.llvm.org/D102688