This patch fixes and eliminates the possibility of SupportFileSP ever
being nullptr. The support file was originally treated like a value
type, but became a polymorphic type and therefore has to be stored and
passed around as a pointer.

To avoid having all the callers check the validity of the pointer, I
introduced the invariant that SupportFileSP is never null and always
default constructed. However, without enforcement at the type level,
that's fragile and indeed, we already identified two crashes where
someone accidentally broke that invariant.

This PR introduces a NonNullSharedPtr to prevent that. NonNullSharedPtr
is a smart pointer wrapper around std::shared_ptr that guarantees the
pointer is never null. If default-constructed, it creates a
default-constructed instance of the contained type. Note that I'm using
private inheritance because you shouldn't inherit from standard library
classes due to the lack of virtual destructor. So while the new
abstraction looks like a `std::shared_ptr`, it is in fact **not** a
shared pointer. Given that our destructor is trivial, we could use
public inheritance, but currently there's no need for it.

rdar://164989579

This fixes a few bugs, effectively through a fallback to `p` when `po` fails.

The motivating bug this fixes is when an error within the compiler causes `po` to fail.
Previously when that happened, only its value (typically an object's address) was
printed – and problematically, no compiler diagnostics were shown. With this change,
compiler diagnostics are shown, _and_ the object is fully printed (ie `p`).

Another bug this fixes is when `po` is used on a type that doesn't provide an object
description (such as a struct). Again, the normal `ValueObject` printing is used.

Additionally, this also improves how lldb handles an object description method that
fails in some way. Now an error will be shown (it wasn't before), and the value will be
printed normally.

This patch improves the synchronization of the debugger's output and error
streams using two new abstractions: `LockableStreamFile` and
`LockedStreamFile`.

 - `LockableStreamFile` is a wrapper around a `StreamFile` and a mutex. Client
   cannot use the `StreamFile` without calling `Lock`, which returns a
   `LockedStreamFile`.

 - `LockedStreamFile` is an RAII object that locks the stream for the duration
   of its existence.  As long as you hold on to the returned object you are
   permitted to write to the stream. The destruction of the object
   automatically flush the output stream.

Prior to this patch, the function returned an exit status, sometimes a
ValueObject with an error and a Status object. This patch removes the
Status object and ensures the error is consistently returned as the
error of the ValueObject.

To support detecting MD5 checksum mismatches, deal with SupportFiles
rather than a plain FileSpecs in the SourceManager.

This patch replaces uses of StringRef::{starts,ends}with with
StringRef::{starts,ends}_with for consistency with
std::{string,string_view}::{starts,ends}_with in C++20.

I'm planning to deprecate and eventually remove
StringRef::{starts,ends}with.

This method was working as expected if LLDB_ENABLE_LIBEDIT is false, however, if it was true, then the variable m_current_lines_ptr was always pointing to an empty list, because Editline only updates its contents once the full input has been completed (see https://github.com/llvm/llvm-project/blob/main/lldb/source/Host/common/Editline.cpp#L1576).

A simple fix is to invoke Editline::GetInputAsStringList() from GetCurrentLines(), which is already used in many places as the common way to get the full input list.

StreamFile subclasses Stream (from lldbUtility) and is backed by a File
(from lldbHost). It does not depend on anything from lldbCore or any of its
sibling libraries, so I think it makes sense for this to live in
lldbHost instead.

Differential Revision: https://reviews.llvm.org/D157460

None of these need to be in the ConstString StringPool. For the most
part they are constant strings and do not require fast comparisons.

I did change IOHandlerDelegateMultiline slightly -- specifically, the
`m_end_line` member always has a `\n` at the end of it now. This was so
that `IOHandlerGetControlSequence` can always return a StringRef. This
did require a slight change to `IOHandlerIsInputComplete` where we must
drop the newline before comparing it against the input parameter.

Differential Revision: https://reviews.llvm.org/D151597

These don't really need to be in the ConstString StringPool. I've
changed the return type to StringRef because on llvm.org and downstream
in the swift fork, this returns a constant value. We could change it to
return a std::string or something else if it needs to be able to change
between calls.

Differential Revision: https://reviews.llvm.org/D151962