Set `incremental = true` for builds that don't output LLVM IR

@kangalioo, @danielhenrymantilla and I have been trying to figure out how to use the playground to run procedural macros, for a Discord bot. The code so far looks like this:

fn main(){let _=std::process::Command::new("/bin/bash").args(&["-c", r##"
set -euo pipefail
cat >>Cargo.toml <<EOF
    [lib]
    proc-macro = true
EOF
cat >src/lib.rs <<EOF
    use proc_macro::TokenStream;

    #[proc_macro]
    pub fn foo(input: TokenStream) -> TokenStream {
        quote::ToTokens::into_token_stream(syn::parse_macro_input!(input as syn::Item)).into()
    }
EOF
cat >src/main.rs <<EOF
    ::playground::foo!(fn main() {});
EOF

cargo run --offline
"##]).status();}

However, this doesn't work because all the dependencies used in the playground get rebuilt and the process gets timed out. This happens because incremental = false is enabled for all builds, a change made in #259 to fix the output of the "Show LLVM IR" command. If incremental = false was disabled for all builds except those "Show LLVM IR" ones, this code would work.

Start by locating the build configuration that sets incremental compilation and the code path for the “Show LLVM IR” command. Compare those build modes, then verify that ordinary builds use incremental compilation without changing the LLVM IR output; test the procedural-macro scenario described in the issue.