Catch assumes std::uncaught_exceptions is available whenever C++17 is
available, but for macOS versions older than 10.12 this is not the case.

Instead of checking the C++ version, use a macro to check whether the
feature is available.

CATCH_INTERNAL_IGNORE_BUT_WARN() introduced with b7b346c triggers
clang-tidy warning 'cppcoreguidelines-pro-type-vararg' for every usage
of assertion macros like CHECK() and REQUIRE(). Silence it via NOLINT
in the '#if defined(__clang__)' block only, as clang-tidy honors those.

This is a partial revert of b7b346c3e5603.

The old code caused warnings to fire under MSVC, and Clang <3.8.
I could not find a GCC version where it worked, but I assume that it
did at some point.

This new code causes all of MSVC, GCC, Clang, in current versions,
to emit signed/unsigned comparison warning in test like this:

```cpp
TEST_CASE() {
    int32_t i = -1;
    uint32_t j = 1;
    REQUIRE(i != j);
}
```

Where previously only MSVC would emit the warning.

Fixes #1880

This is both a really big and a really small commit. It is small in
that it only contains renaming, moving and modification of include
directives caused by this.

It is really big in the obvious way of touching something like 200
files.

The new rules for naming files is simple: headers use the `.hpp`
extension. The rules for physical file layout is still kinda in
progress, but the basics are also simple:
 * Significant parts of functionality get their own subfolder
   * Benchmarking is in `catch2/benchmark`
   * Matchers are in `catch2/matchers`
   * Generators are in `catch2/generators`
   * Reporters are in `catch2/reporters`
   * Baseline testing facilities are in `catch2/`
 * Various top level folders also contain `internal` subfolder,
   with files that users probably do not want to include directly,
   at least not until they have to write something like their own
   reporter.
    * The exact files in these subfolders is likely to change later
      on

Note that while some includes were cleaned up in this commit, it
is only the low hanging fruit and further cleanup using automatic
tooling will happen later.

Also note that various include guards, copyright notices and file
headers will also be standardized later, rather than in this commit.