message: Update split marker for split-input-file option to be more restrictive
  This is to allow usage of comment blocks along with splits in test cases.
  For example, "Function Control Flow Lowering" comment block in
  raise-control-flow.mlir
  TESTED with existing unit tests
diff: @@ -332,7 +332,7 @@ static OptResult processFile(std::unique_ptr<MemoryBuffer> ownedBuffer) {
  /// single test, but could be used for other purposes as well.
  static OptResult
  splitAndProcessFile(std::unique_ptr<MemoryBuffer> originalBuffer) {
  - const char marker[] = "-----";
  + const char marker[] = "// -----\n";
  auto *origMemBuffer = originalBuffer.get();
  SmallVector<StringRef, 8> sourceBuffers;
  origMemBuffer->getBuffer().split(sourceBuffers, marker);
  