Unrolled build for rust-lang#123294

Rollup merge of rust-lang#123294 - Nilstrieb:reuqire-llvm-config, r=clubby789

Require LLVM_CONFIG to be set in rustc_llvm/build.rs

This environment variable should always be set by bootstrap in `rustc_llvm_env`. The fallback is quite ugly and complicated, so removing it is nice.

https://github.com/rust-lang/rust/blob/bf71daedc29e7a240261acd1516378047e311a6f/src/bootstrap/src/core/build_steps/compile.rs#L1166

I tried finding when this was added in git history, but it pointed all the way to "add build scripts" at which point I stopped digging more. This has always been here.

cc `@nikic` `@cuviper` in case you happen to be aware of a deeper reason behind this

r? bootstrap

Author: rust-timer

compiler/rustc_llvm/build.rs

@@ -112,28 +112,10 @@ fn main() {
 
     restore_library_path();
 
-    let target = env::var("TARGET").expect("TARGET was not set");
     let llvm_config =
-        tracked_env_var_os("LLVM_CONFIG").map(|x| Some(PathBuf::from(x))).unwrap_or_else(|| {
-            if let Some(dir) = tracked_env_var_os("CARGO_TARGET_DIR").map(PathBuf::from) {
-                let to_test = dir
-                    .parent()
-                    .unwrap()
-                    .parent()
-                    .unwrap()
-                    .join(&target)
-                    .join("llvm/bin/llvm-config");
-                if Command::new(&to_test).output().is_ok() {
-                    return Some(to_test);
-                }
-            }
-            None
-        });
+        PathBuf::from(tracked_env_var_os("LLVM_CONFIG").expect("LLVM_CONFIG was not set"));
 
-    if let Some(llvm_config) = &llvm_config {
-        println!("cargo:rerun-if-changed={}", llvm_config.display());
-    }
-    let llvm_config = llvm_config.unwrap_or_else(|| PathBuf::from("llvm-config"));
+    println!("cargo:rerun-if-changed={}", llvm_config.display());
 
     // Test whether we're cross-compiling LLVM. This is a pretty rare case
     // currently where we're producing an LLVM for a different platform than