[lldb] Add a simplified syntax for underlying command options (NFC) (#155694)
This PR updates the tablegen emitter for command options to support a
simplified syntax to underline the mnemonic. Previously, you had to
write `${ansi.underline}<L>${ansi.normal}`, where `<L>` is the mnemonic.
This really hurt the readability of the description. With this PR, you
can write `${<L>}` instead.