llvm-builder: do not build clang by default (#168)

We only need LLD for cross compilation. This significantly reduces the LLVM build times in a cross compilation scenario. Update the README as a drive-by.

crates/llvm-builder/Cargo.toml

@@ -19,7 +19,7 @@ path = "src/revive_llvm/main.rs"
 doctest = false
 
 [dependencies]
-clap = { workspace = true, features = ["std", "derive"] }
+clap = { workspace = true, features = ["help", "std", "derive"] }
 anyhow = { workspace = true }
 serde = { workspace = true, features = [ "derive" ] }
 toml = { workspace = true }

crates/llvm-builder/README.md

@@ -83,7 +83,7 @@ Obtain a compatible build for your host platform from the release section of thi
 
       ```shell
       revive-llvm clone
-      revive-llvm build
+      revive-llvm build --llvm-projects lld --llvm-projects clang
       ```
 
       Build artifacts end up in the `./target-llvm/gnu/target-final/` directory by default.

crates/llvm-builder/src/revive_llvm/arguments.rs

@@ -36,13 +36,7 @@ pub enum Subcommand {
         targets: Vec<String>,
 
         /// LLVM projects to build LLVM with.
-        #[arg(
-            long,
-            default_values_t = vec![
-                revive_llvm_builder::llvm_project::LLVMProject::CLANG,
-                revive_llvm_builder::llvm_project::LLVMProject::LLD
-            ]
-        )]
+        #[arg(long)]
         llvm_projects: Vec<revive_llvm_builder::llvm_project::LLVMProject>,
 
         /// Whether to build LLVM with run-time type information (RTTI) enabled.