`emit-cir-flat` is a bit broken

[bcardosolopes]

Driver isn't doing the proper job of giving precedence to the last specified one (and the message is also pretty broken).

clang++ -std=c++20 -target arm64-apple-macosx14.0.0 -Wno-unused-value -fexceptions -c lambda.cpp -Xclang -fclangir -Xclang -emit-cir -o lambda.cir -Xclang -emit-cir-flat
error: '-emit-cir' action ignored; '-emit-cir-flat' action specified previously