Add a hack to prevent proc_macro misopt in CI

Author: nbdd0121

library/proc_macro/src/bridge/buffer.rs

@@ -10,8 +10,10 @@ pub struct Buffer {
     data: *mut u8,
     len: usize,
     capacity: usize,
-    reserve: extern "C" fn(Buffer, usize) -> Buffer,
-    drop: extern "C" fn(Buffer),
+    // HACK(nbdd0121): These should be `extern "C"` but LLVM < 17.0.4 will misoptimise if we do so.
+    // Remove this when we drop support for them or when we require patches to be present.
+    reserve: extern "C-unwind" fn(Buffer, usize) -> Buffer,
+    drop: extern "C-unwind" fn(Buffer),
 }
 
 unsafe impl Sync for Buffer {}
@@ -141,13 +143,13 @@ impl From<Vec<u8>> for Buffer {
             }
         }
 
-        extern "C" fn reserve(b: Buffer, additional: usize) -> Buffer {
+        extern "C-unwind" fn reserve(b: Buffer, additional: usize) -> Buffer {
             let mut v = to_vec(b);
             v.reserve(additional);
             Buffer::from(v)
         }
 
-        extern "C" fn drop(b: Buffer) {
+        extern "C-unwind" fn drop(b: Buffer) {
             mem::drop(to_vec(b));
         }