compiletest: don't register MSVC/NONMSVC FileCheck prefixes

This was fragile as it was based on host target passed to compiletest,
but the user could cross-compile and run test for a different target
(e.g. cross from linux to msvc, but msvc won't be set on the target).
Furthermore, it was also very surprising as normally revision names
(other than `CHECK`) was accepted as FileCheck prefixes.

Author: jieyouxu

src/tools/compiletest/src/runtest.rs

@@ -1958,23 +1958,36 @@ impl<'test> TestCx<'test> {
         let mut filecheck = Command::new(self.config.llvm_filecheck.as_ref().unwrap());
         filecheck.arg("--input-file").arg(output).arg(&self.testpaths.file);
 
-        // FIXME: Consider making some of these prefix flags opt-in per test,
-        // via `filecheck-flags` or by adding new header directives.
-
         // Because we use custom prefixes, we also have to register the default prefix.
         filecheck.arg("--check-prefix=CHECK");
 
-        // Some tests use the current revision name as a check prefix.
+        // FIXME(jieyouxu): the current scheme of conflating `compiletest` revisions with
+        // `FileCheck` prefixes is questionable. The current scheme means that `compiletest`
+        // revision names **implicitly** registers a `FileCheck` prefix of the same name. But this
+        // is questionable: with this setup, there is no way to distinguish whether the test writer
+        // wants to:
+        //
+        // 1. Use revisions but ONLY for compiletest directive purposes. E.g. `FileCheck`
+        //    annotations should ONLY be using the default `CHECK` prefix.
+        // 2. Use revisions but ONLY for filecheck prefix purposes. This use case is somewhat
+        //    questionable; previously we implicitly injected `MSVC` for msvc host and `NONMSVC` for
+        //    non-msvc host, but that is problematic in cross-compile test scenarios.
+        // 3. Use revisions for BOTH. This conflates `compiletest` directives (e.g. conditional test
+        //    execution) and `FileCheck` directives, and tthet test writer can't really separate
+        //    them.
+        //
+        // A better solution might be to split these probably-orthogonal concepts, and use a
+        // dedicated directive like `//@ filecheck-prefixes: MSVC NONMSVC` to not conflate
+        // `compiletest` directives with `FileCheck` prefixes.
+
+        // HACK: tests are allowed to use a revision name as a check prefix.
         if let Some(rev) = self.revision {
             filecheck.arg("--check-prefix").arg(rev);
         }
 
-        // Some tests also expect either the MSVC or NONMSVC prefix to be defined.
-        let msvc_or_not = if self.config.target.contains("msvc") { "MSVC" } else { "NONMSVC" };
-        filecheck.arg("--check-prefix").arg(msvc_or_not);
-
-        // The filecheck tool normally fails if a prefix is defined but not used.
-        // However, we define several prefixes globally for all tests.
+        // HACK: the filecheck tool normally fails if a prefix is defined but not used. However,
+        // sometimes revisions are used to specify *compiletest* directives which are not FileCheck
+        // concerns.
         filecheck.arg("--allow-unused-prefixes");
 
         // Provide more context on failures.