Not being able to filter generators to specific element has been regularly
causing problems. It was possible to use a dynamic section to run tests
for specific element in a generator, at least if the element had a nice
string representation, but the test case would still run once per element
in the generator.

With this change, it is possible to have the generator return only one
specific element, and do so based on the index, rather than the string
representation of the element. This enables simple debugging of tests
that fail for specific generator element.

MSVC cannot dllexport thread_local variables, so we avoid making
globals thread local if we won't support multiple threads anyway.

Closes #3044

All the previous refactoring to make the assertion fast paths
smaller and faster also allows us to implement the fast paths
just with thread-local and atomic variables, without full mutexes.

However, the performance overhead of thread-safe assertions is
still significant for single threaded usage:

|  slowdown |  Debug  | Release |
|-----------|--------:|--------:|
| fast path |   1.04x |   1.43x |
| slow path |   1.16x |   1.22x |

Thus, we don't make the assertions thread-safe by default, and instead
provide a build-time configuration option that the users can set to get
thread-safe assertions.

This commit is functional, but it still needs some follow-up work:
 * We do not need full seq_cst increments for the atomic counters,
   and using weaker ones can be faster.
 * We brute-force updating the reporter-friendly totals from internal
   atomic counters by doing it everywhere. We should properly trace
   where this is needed instead.
 * Message macros (`INFO`, `UNSCOPED_INFO`, `CAPTURE`, etc) are not
   made thread safe in this commit, but they can be made thread safe
   in the future, by building on top of this work.
 * Add more tests, including with thread-sanitizer, and compiled
   examples to the repository. Right now, these changes have been
   compiled with tsan manually, but these tests are not added to CI.

Closes #2948

Adding unreachable to `FAIL` and `SKIP` made Clang become very good
at figuring out that tests of `FAIL` and `SKIP` will never return
and thus could be marked as [[noreturn]].

To avoid introducing lot of warning suppression noise into the
test files, we just disable it instead.

This helps the compiler see that the execution does not continue,
which in turn means that it is not an error if the user does not
return value from the function after FAIL/SKIP.

Closes #2941

Move `catch_test_run_info.hpp` from directory
`src/catch2/internal/` to be directly under `src/catch2/`.

Header `catch_test_run_info.hpp` defines `Catch::TestRunInfo` which is part of the public API, since it is required when overriding `Catch::EventListenerBase`.

Fixes issue caught by Clang Tidy misc-include-cleaner check.

2 spaces seems to be the more common indentation level so that's what
I unified around.

Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.