avoid replacing the definition of CURRENT_RUSTC_VERSION

Before this commit, replace-version-placeholder hardcoded the path
defining CURRENT_RUSTC_VERSION (to avoid replacing it). After a refactor
moved the file defining it without changing the hardcoded path, the tool
started replacing the constant itself with the version number.

To avoid this from happening in the future, this changes the definition
of the constant to avoid the tool from ever matching it.

src/tools/replace-version-placeholder/src/main.rs

@@ -11,12 +11,7 @@ fn main() {
     let version_str = version_str.trim();
     walk::walk_many(
         &[&root_path.join("compiler"), &root_path.join("library")],
-        |path, _is_dir| {
-            walk::filter_dirs(path)
-                // We exempt these as they require the placeholder
-                // for their operation
-                || path.ends_with("compiler/rustc_attr/src/builtin.rs")
-        },
+        |path, _is_dir| walk::filter_dirs(path),
         &mut |entry, contents| {
             if !contents.contains(VERSION_PLACEHOLDER) {
                 return;