and other ASSERT usage changes

Per https://en.cppreference.com/w/cpp/preprocessor/replace#.23_and_.23.23_operators the `##` behavior is a nonstandard extension; this extension seems to be supported by all compilers we care about, but IntelliSense in visual studio doesn't correctly handle it, resulting in false errors in the IDE (but not when compiling).

Per https://en.cppreference.com/w/cpp/preprocessor/replace#Function-like_macros C++20 introduced a workaround, where `__VA_OPT__(, )` generates a comma if and only if `__VA_ARGS__` is non-empty.

This PR replaces all occurrences, with the exception of Externals, DSPSpy (which is not likely to be edited in MSVC and does not target C++20 currently), and JitArm64_Integer.cpp (which uses `Function(__VA_ARGS__)`, and thus does not ever need a comma).

Since it was unused, nonexistent values were used in a few places.  I've replaced them.

This replaces the MAX_LOGLEVEL define with a constexpr variable
in order to fix self-comparison warnings in the logging macros
when compiling with Clang. (Without this change, the log level check
in the logging macros is expanded into something like this:
`if (LINFO <= LINFO)`, which triggers a tautological compare warning.)

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Previously the logging was a in a little bit of a disarray. Some things
were in namespaces, and other things were not.

Given this code will feature a bit of restructuring during the
transition over to fmt, this is a good time to unify it under a single
namespace and also remove functions and types from the global namespace.

Now, all functions and types are under the Common::Log namespace. The
only outliers being, of course, the preprocessor macros.