[LoopInfo][NewPM] Print function name in LoopPrinterPass (#76527)

The legacy pass manager printed the function name when printing
loop info (via -analyze option). Like this:

  Printing analysis 'Natural Loop Information' for function 'func':
  Loop at depth 1 containing: ...
      Loop at depth 2 containing: ...

Make sure we print such a first line including the function name
also when using the new pass manager version of LoopPrinterPass.

The format of the string is changed slightly, so now we say:

  Loop info for function 'func':
  Loop at depth 1 containing: ...
      Loop at depth 2 containing: ...

This was originally requested in
  https://discourse.llvm.org/t/need-usage-help-w-new-pass-manager-for-opt-analysis-natural-loop-information/75874/7
and also mentioned in
  https://github.com/llvm/llvm-project/issues/76762