[libc++][CI] Fixes error messages.

[Mordante]

Summary

Fixes 2 error messages from the module test clang-tidy plugin.

• an issue where an empty filename can't be used in an AST matcher
• an issue where no filename is valid when testing the std module cppm

This reduces the output of the test, making it easier to find the real
error when the test fails.

[philnik]

Could you add a short description of what this suppresses to the commit message?

[Mordante]

In D157741#4592772, @philnik wrote:

Could you add a short description of what this suppresses to the commit message?

Good point, seems I forgot that.

libcxx/test/tools/clang_tidy_checks/header_exportable_declarations.cpp

 
 namespace libcpp {
 header_exportable_declarations::header_exportable_declarations(
     llvm::StringRef name, clang::tidy::ClangTidyContext* context)
     : clang::tidy::ClangTidyCheck(name, context),
       filename_(Options.get("Filename", "")),
       file_type_(Options.get("FileType", header_exportable_declarations::FileType::Unknown)),
       extra_header_(Options.get("ExtraHeader", "")) {
-  if (filename_.empty())
-    llvm::errs() << "No filename is provided.\n";
-
   switch (file_type_) {
   case header_exportable_declarations::FileType::Header:
-    /* DO NOTHING */
+    if (filename_.empty())
+      llvm::errs() << "No filename is provided.\n";
+    if (extra_header_.empty())
+      extra_header_ = "$^"; // Use a never matching regex to silence an error message.
     break;
-  case header_exportable_declarations::FileType::Module:
   case header_exportable_declarations::FileType::ModulePartition:
+    if (filename_.empty())
+      llvm::errs() << "No filename is provided.\n";
+    [[fallthrough]];
+  case header_exportable_declarations::FileType::Module:
     if (!extra_header_.empty())
       llvm::errs() << "Extra headers are not allowed for modules.\n";
     if (Options.get("SkipDeclarations"))
       llvm::errs() << "Modules may not skip declarations.\n";
     if (Options.get("ExtraDeclarations"))
       llvm::errs() << "Modules may not have extra declarations.\n";
     break;
   case header_exportable_declarations::FileType::Unknown: