Add a feature to explicitly include a file's SemIR (#5961)

Trying to figure out an easy way to debug semir in the prelude, #5703
removed an option to set `--exclude-dump-file-prefix` to empty. But,
this is probably an improvement over that flow... With this change, it's
possible to add `//@dump-sem-ir-file` to a specific prelude file, and
its full IR will be printed. Additionally, it becomes an option with the
default `--dump-sem-ir-ranges=only` to add `//@dump-sem-ir-file` and get
the full file's IR.

toolchain/lex/tokenized_buffer.cpp

@@ -236,6 +236,10 @@ auto TokenizedBuffer::Print(llvm::raw_ostream& output_stream,
     output_stream << "\n";
   }
 
+  if (has_include_in_dumps_) {
+    output_stream << "  has_include_in_dumps: true\n";
+  }
+
   if (!dump_sem_ir_ranges_.empty()) {
     output_stream << "  dump_sem_ir_ranges:\n";
     for (auto range : dump_sem_ir_ranges_) {