Check that `library/profiler_builtins` actually found some source files

The current `build.rs` will automatically skip source files that don't exist.
An unfortunate side-effect is that if _no_ files could be found (e.g. because
the directory was wrong), the build fails with a mysterious linker error.

We can reduce the awkwardness of this by explicitly checking that at least one
source file was found.

library/profiler_builtins/build.rs

@@ -84,12 +84,16 @@ fn main() {
     let root = Path::new("../../src/llvm-project/compiler-rt");
 
     let src_root = root.join("lib").join("profile");
+    assert!(src_root.exists(), "profiler runtime source directory not found: {src_root:?}");
+    let mut n_sources_found = 0u32;
     for src in profile_sources {
         let path = src_root.join(src);
         if path.exists() {
             cfg.file(path);
+            n_sources_found += 1;
         }
     }
+    assert!(n_sources_found > 0, "couldn't find any profiler runtime source files in {src_root:?}");
 
     cfg.include(root.join("include"));
     cfg.warnings(false);