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

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.

Reviewed By: philnik, #libc

Differential Revision: https://reviews.llvm.org/D157741

Author: mordante

libcxx/test/tools/clang_tidy_checks/header_exportable_declarations.cpp

@@ -38,15 +38,18 @@ header_exportable_declarations::header_exportable_declarations(
       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"))