Add a failing test for Clang modules support. (#7233)

Rearrange the file_test infrastructure so that we can customize the
mapping of file names to command line arguments. Map `module.modulemap`
files to corresponding Clang driver flags. In passing, also clean up the
interface for specifying custom argument replacements so that we don't
build a string map for each file we process, and stop using
`SmallVector::insert`.

Assisted-by: Gemini via Antigravity

testing/file_test/file_test_base_test.cpp

@@ -31,14 +31,25 @@ class FileTestBaseTest : public FileTestBase {
            llvm::raw_pwrite_stream& error_stream) const
       -> ErrorOr<RunResult> override;
 
-  auto GetArgReplacements() const -> llvm::StringMap<std::string> override {
-    return {{"replacement", "replaced"}};
-  }
-
   auto GetDefaultArgs() const -> llvm::SmallVector<std::string> override {
     return {"default_args", "%s"};
   }
 
+  auto AddArgsForFilename(llvm::SmallVectorImpl<std::string>& args,
+                          llvm::StringRef filename) const -> void override {
+    if (!filename.ends_with(".exclude")) {
+      args.emplace_back(filename);
+    }
+  }
+
+  auto GetArgReplacement(llvm::StringRef key) const
+      -> std::optional<std::string> override {
+    if (key == "replacement") {
+      return "replaced";
+    }
+    return std::nullopt;
+  }
+
   auto GetDefaultFileRE(llvm::ArrayRef<llvm::StringRef> filenames) const
       -> std::optional<RE2> override {
     return std::make_optional<RE2>(