Add lexing for dump-sem-ir-start and end (#5357)

Syntax rationale is on `DumpSemIRRange` to try and record this, since
I'm not sure this belongs in the language design. The intent of this is
to be able to subset SemIR, which will be done separately in the
formatter.

---------

Co-authored-by: David Blaikie <dblaikie@gmail.com>

toolchain/lex/tokenized_buffer.cpp

@@ -236,6 +236,14 @@ auto TokenizedBuffer::Print(llvm::raw_ostream& output_stream,
     PrintToken(output_stream, token, widths);
     output_stream << "\n";
   }
+
+  if (!dump_sem_ir_ranges_.empty()) {
+    output_stream << "  dump_sem_ir_ranges:\n";
+    for (auto range : dump_sem_ir_ranges_) {
+      output_stream << "  - {start: " << range.start.index
+                    << ", end: " << range.end.index << "}\n";
+    }
+  }
 }
 
 auto TokenizedBuffer::PrintToken(llvm::raw_ostream& output_stream,