[llvm-dev] Binary utilities: switch command line parsing from llvm::cl to OptTable (byproduct: drop -long-option?)

[Fāng-ruì Sòng via llvm-dev]

llvm/tools/ include some binary utilities used as replacement for GNU
binutils, e.g. llvm-objcopy, llvm-symbolizer, llvm-nm.
In some old threads people discussed some drawbacks of using cl::opt for
user-facing utilities (I cannot find them now).
Switching to OptTable is an appealing solution. I have prepared two patches
for two binary utilities: llvm-nm and llvm-strings.

* llvm-strings https://reviews.llvm.org/D104889
* llvm-nm https://reviews.llvm.org/D105330

llvm-symbolizer was switched last year. llvm-objdump was switched by thakis
earlier this year.

The switch can fix some corners with lib/Support/CommandLine.cpp. Here is a
summary:

* -t=d is removed (equal sign after a short option). Use -t d instead.
* --demangle=0 (=0 to disable a boolean option) is removed. Omit the option
or use --no-demangle instead.
* To support boolean options (e.g. --demangle --no-demangle), we don't need
to compare their positions (if (NoDemangle.getPosition() >
Demangle.getPosition()) , see llvm-nm.cpp)
* grouped short options can be specified with one line
`setGroupedShortOptions`, instead of adding cl::Grouping to every short
options.
* We don't need to add cl::cat to every option and call
`HideUnrelatedOptions` to hide unrelated options from --help. The issue
would happen with cl::opt tools if linker garbage collection is disabled or
libLLVM-13git.so is used. (See https://reviews.llvm.org/D104363)
* If we decide to support binary utility multiplexting (
https://reviews.llvm.org/D104686), we will not get conflicting options. An
option may have different meanings in different utilities (especially for
one-letter options).

*I expect that most users will not observe any difference.*

There is a related topic whether we should disallow the single-dash
`-long-option` form.
(Discussed in 2019:
https://lists.llvm.org/pipermail/llvm-dev/2019-April/131786.html Accept
--long-option but not -long-option for llvm binary utilities)
*I'd like to disallow -long-option but may want to do this in a separate
change.*
The main point is that (1) grouped short options have syntax conflict with
one-dash long options. (2) the GNU getopt_long style two-dash long option
is much more popular.

I can think of potential pushback for some Mach-O specific options, e.g. nm
-arch
http://www.manpagez.com/man/1/nm/osx-10.12.6.php says `-arch` has one dash.
If such options may have problems, we can keep supporting one dash forms.
With OptTable, allowing one-dash forms for a specific option is easy.
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