Revert "Temporarily disable nightly testing due to libLLVM link issue"

This reverts commit 7aef1ed.

build.rs

@@ -11,9 +11,7 @@ fn main() {
     // Note: add "/build.rs" to package.include in Cargo.toml if adding any
     // conditional compilation within the library.
 
-    if !unstable() || {
-        true // FIXME: waiting on https://github.com/rust-lang/rust/pull/121967
-    } {
+    if !unstable() {
         println!("cargo:rustc-cfg=syn_disable_nightly_tests");
     }
 }