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

This simple diff declares some enum values needed to create a REPL for the mojo language.

Differential Revision: https://reviews.llvm.org/D150303

REPL implementations don't have an easy way to know that an expression has been evaluated, so I'm adding a simple function for that. In the future we can add another hook for meta commands.

Differential Revision: https://reviews.llvm.org/D149719