Adds a `--defaults`/`-d` flag to `settings show`. This mode will _optionally_ show a
setting's default value. In other words, this does not always print a default value for
every setting.

A default value is not shown when the current value _is_ the default.

Note: some setting types do not print empty or invalid values. For these setting types,
if the default value is empty or invalid, the same elision logic is applied to printing
the default value.

Completes the ToJSON function for `OptionValue` types and make the interface function pure virtual

---------

Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>

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.

Make the `Get.*Value` and `Set.*Value` function private and migrate the
last remaining call sites to the new overloaded/templated functions.

Various OptionValue related classes are passing around will_modify but
the value is never used. This patch simplifies the interfaces by
removing the redundant argument.

Refactor OptionValue to return a std::optional instead of taking a fail
value. This allows the caller to handle situations where there's no
value, instead of being unable to distinguish between the absence of a
value and the value happening the match the fail value. When a fail
value is required, std::optional::value_or() provides the same
functionality.

This patch adds new SBDebugger::GetSetting() API which
enables client to access settings as SBStructedData.

Implementation wise, a new ToJSON() virtual function is added to OptionValue
class so that each concrete child class can override and provides its
own JSON representation. This patch aims to define the APIs and implement
a common set of OptionValue child classes, leaving the remaining for
future patches.

This patch is used later by auto deduce source map from source line breakpoint
feature for testing generated source map entries.

Differential Revision: https://reviews.llvm.org/D133038

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D130403

Replace misc. StringConvert uses with llvm::to_integer()
and llvm::to_float(), except for cases where further refactoring is
planned.  The purpose of this change is to eliminate the StringConvert
API that is duplicate to LLVM, and less correct in behavior at the same
time.

Differential Revision: https://reviews.llvm.org/D110447