compiletest: add max-llvm-major-version directive

There's already `min-llvm-version`, and contributors were using
`ignore-llvm-version: 20 - 99` to emulate `max-llvm-major-version: 19`.

Author: jieyouxu

src/tools/compiletest/src/directive-list.rs

@@ -120,6 +120,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "incremental",
     "known-bug",
     "llvm-cov-flags",
+    "max-llvm-major-version",
     "min-cdb-version",
     "min-gdb-version",
     "min-lldb-version",

src/tools/compiletest/src/header.rs

@@ -1546,6 +1546,20 @@ fn ignore_llvm(config: &Config, line: &str) -> IgnoreDecision {
                     ),
                 };
             }
+        } else if let Some(version_string) =
+            config.parse_name_value_directive(line, "max-llvm-major-version")
+        {
+            let max_version = extract_llvm_version(&version_string);
+            // Ignore if actual major version is larger than the maximum required major version.
+            if actual_version.major > max_version.major {
+                return IgnoreDecision::Ignore {
+                    reason: format!(
+                        "ignored when the LLVM version ({actual_version}) is newer than major\
+                        version {}",
+                        max_version.major
+                    ),
+                };
+            }
         } else if let Some(version_string) =
             config.parse_name_value_directive(line, "min-system-llvm-version")
         {

src/tools/compiletest/src/header/tests.rs

@@ -299,6 +299,15 @@ fn llvm_version() {
 
     let config: Config = cfg().llvm_version("10.6.2").build();
     assert!(!check_ignore(&config, "//@ exact-llvm-major-version: 10"));
+
+    let config: Config = cfg().llvm_version("19.0.0").build();
+    assert!(!check_ignore(&config, "//@ max-llvm-major-version: 19"));
+
+    let config: Config = cfg().llvm_version("19.1.2").build();
+    assert!(!check_ignore(&config, "//@ max-llvm-major-version: 19"));
+
+    let config: Config = cfg().llvm_version("20.0.0").build();
+    assert!(check_ignore(&config, "//@ max-llvm-major-version: 19"));
 }
 
 #[test]