Add warning to the Builder::llvm_config function

(cherry picked from commit 6315b49)

Author: Kobzol

src/bootstrap/src/core/builder/mod.rs

@@ -1613,6 +1613,10 @@ You have to build a stage1 compiler for `{}` first, and then use it to build a s
     ///
     /// Note that this returns `None` if LLVM is disabled, or if we're in a
     /// check build or dry-run, where there's no need to build all of LLVM.
+    ///
+    /// FIXME(@kobzol)
+    /// **WARNING**: This actually returns the **HOST** LLVM config, not LLVM config for the given
+    /// *target*.
     pub fn llvm_config(&self, target: TargetSelection) -> Option<PathBuf> {
         if self.config.llvm_enabled(target) && self.kind != Kind::Check && !self.config.dry_run() {
             let llvm::LlvmResult { host_llvm_config, .. } = self.ensure(llvm::Llvm { target });