Log errors to the (always-on) system log if they would otherwise get
dropped by LLDB_LOG_ERROR.

Add an "always on" log category and channel. Unlike other, existing log
channels, it is not exposed to users. The channel is meant to be used
sparsely and deliberately for logging high-value information to the
system log.

We have a similar concept in the downstream Swift fork and this has
proven to be extremely valuable. This is especially true on macOS where
system log messages are automatically captured as part of a sysdiagnose.

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.

Add a T-style log handler that multiplexes messages to two log handlers.
The goal is to use this in combination with the SystemLogHandler to log
messages both to the user requested file as well as the system log. The
latter is part of a sysdiagnose on Darwin which is commonly attached to
bug reports.

Uses of (void) remain where they are for purposes other than an
assert variable.

LLDB's logging machinery supports prepending log messages with the name
of the file and function that generates the log. However, currently this
functionality is limited to the LLDB_LOG macro. I meant to do this as a
follow up to D65128 but never got around to it.

Differential revision: https://reviews.llvm.org/D151764

* As no format string is involved, avoid unecessary call into `Printf`
* Eliminate creation of a `std::string` to print a `StringRef`

Differential Revision: https://reviews.llvm.org/D150160

Instead of creating a std::string from the `SmallString`,
let's just use a std::string from the start. I initially tried to make
`SmallString` work but getting it right proved complicated because
`LogHandler::Emit` will take its `StringRef` parameter and touch the raw
`const char *` from it directly, which isn't guaranteed to be
null-terminated with a `SmallString`.

I changed `WriteMessage` to take a `StringRef` instead of a
`const std::string &` for flexibility.

Differential Revision: https://reviews.llvm.org/D148676

In January, Greg put up a patch (D117382) to support, among other
things, more than 32 log categories. That led to a bunch of nice
cleanups, but categories remained constrained because different parts of
the code were still using uint32_t. This patch fixes the remaining
issues and makes it possible to add a 32nd log category.

Differential revision: https://reviews.llvm.org/D134245

Add a log dump command to dump logs to a file. This only works for
channels that have a log handler associated that supports dumping. For
now that's limited to the circular log handler, but more could be added
in the future.

Differential revision: https://reviews.llvm.org/D128557