std::string::find accepts anything that can be converted to
std::string_view starting in C++17.  Since StringRef can be converted
to std::string_view, we do not need to create a temporary instance of
std::string here.

Use `llvm::Error` instead of `CommandReturnObject` for error reporting.
The command return objects were populated with errors but never
displayed. With this patch they're at least logged.

and implement them for dwim-print (a.k.a. `p`) as an example.

The next step will be to expose them as structured data in
SBCommandReturnObject.

If your arguments or option values are of a type that naturally uses one
of our common completion mechanisms, you will get completion for free.
But if you have your own custom values or if you want to do fancy things
like have `break set -s foo.dylib -n ba<TAB>` only complete on symbols
in foo.dylib, you can use this new mechanism to achieve that.

As specified in the docs,
1) raw_string_ostream is always unbuffered and
2) the underlying buffer may be used directly

( 65b13610a5226b84889b923bae884ba395ad084d for further reference )

* Don't call raw_string_ostream::flush(), which is essentially a no-op.
* Avoid unneeded calls to raw_string_ostream::str(), to avoid excess
indirection.

This patch removes all of the Set.* methods from Status.

This cleanup is part of a series of patches that make it harder use the
anti-pattern of keeping a long-lives Status object around and updating
it while dropping any errors it contains on the floor.

This patch is largely NFC, the more interesting next steps this enables
is to:
1. remove Status.Clear()
2. assert that Status::operator=() never overwrites an error
3. remove Status::operator=()

Note that step (2) will bring 90% of the benefits for users, and step
(3) will dramatically clean up the error handling code in various
places. In the end my goal is to convert all APIs that are of the form

`    ResultTy DoFoo(Status& error)
`
to

`    llvm::Expected<ResultTy> DoFoo()
`
How to read this patch?

The interesting changes are in Status.h and Status.cpp, all other
changes are mostly

` perl -pi -e 's/\.SetErrorString/ = Status::FromErrorString/g' $(git
grep -l SetErrorString lldb/source)
`
plus the occasional manual cleanup.

The dead code is caught by PVS studio analyzer -
https://pvs-studio.com/en/blog/posts/cpp/1126/, fragment N12.

Warning message -
V523 The 'then' statement is equivalent to the 'else' statement.
Options.cpp 1212

For the significant amount of call sites that want to create an
incontrovertible error, such a wrapper function creates a significant
readability improvement and lowers the cost of entry to add error
handling in more places.

I have been looking to simplify parsing logic and improve the interfaces
so that they are both easier to use and harder to abuse. To be specific,
I am referring to functions such as `OptionArgParser::ToBoolean`: I
would like to go from its current interface to something more like
`llvm::Error<bool> ToBoolean(llvm::StringRef option_arg)`.

Through working on that, I encountered 2 inconveniences:
1. Option parsing code is not uniform. Every function writes a slightly
different error message, so incorporating an error message from the
`ToBoolean` implementation is going to be laborious as I figure out what
exactly needs to change or stay the same.
2. Changing the interface of `ToBoolean` would require a global atomic
change across all of the Command code. This would be quite frustrating
to do because of the non-uniformity of our existing code.

To address these frustrations, I think it would be easiest to first
standardize the error reporting mechanism when parsing options in
commands. I do so by introducing `CreateOptionParsingError` which will
create an error message of the shape:
Invalid value ('${option_arg}') for -${short_value} ('${long_value}'):
${additional_context}

Concretely, it would look something like this:
(lldb) breakpoint set -n main -G yay
error: Invalid value ('yay') for -G (auto-continue): Failed to parse as
boolean

After this, updating the interfaces for parsing the values themselves
should become simpler. Because this can be adopted incrementally, this
should be able to done over the course of time instead of all at once as
a giant difficult-to-review change. I've changed exactly one function
where this function would be used as an illustration of what I am
proposing.