[OptTable] Refine how `printHelp` treats empty help texts

Currently, `printHelp` behaves differently for options that:
  * do not define `HelpText` (such options _are not printed_), and
  * define its `HelpText` as `HelpText<"">` (such options _are printed_).
In practice, both approaches lead to no help text and `printHelp` should
treat them consistently. This patch addresses that by making
`printHelpt` check the length of the help text to be printed.

All affected tests have been updated accordingly. The option definitions
for llvm-cvtres have been updated with a short description or "Not
  implemented" for options that are ignored by the tool.

Differential Revision: https://reviews.llvm.org/D107557