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.

Most users of this stick it into a StringRef. The one user that doesn't
just tries to get the length out of it, which we can precompute by
putting it in a constexpr StringLiteral.

Differential Revision: https://reviews.llvm.org/D151951

In the interest of keeping the ConstString StringPool small, this patch
aims to remove the use of ConstString from OptionValueProperties.

We can maintain quick lookups by using an llvm::StringMap to find the
correct index by name.

Differential Revision: https://reviews.llvm.org/D152210

In many places we're using uint32_t where we should be using size_t.
We should be consistent.

Differential Revision: https://reviews.llvm.org/D151949

Use the templated GetPropertyAtIndexAs helper for ArchSpec.

Use the templated GetPropertyAtIndexAs helper for LanguageType.

This patch is a continuation of 6f8b33f6dfd0 and eliminates the
{Get,Set}PropertyAtIndexAsFileSpec functions.

After 6f8b33f6dfd0 this function has no callers anymore.