Introduce combined TUs for compiling small TUs

This should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.

There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.

src/catch2/generators/catch_generator_exception.cpp

@@ -1,14 +0,0 @@
-/*
- *  Distributed under the Boost Software License, Version 1.0. (See accompanying
- *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
- */
-
-#include <catch2/generators/catch_generator_exception.hpp>
-
-namespace Catch {
-
-    const char* GeneratorException::what() const noexcept {
-        return m_msg;
-    }
-
-} // end namespace Catch