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/internal/catch_polyfills.cpp

@@ -1,31 +0,0 @@
-/*
- *  Created by Martin on 17/11/2017.
- *
- *  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/internal/catch_polyfills.hpp>
-
-#include <cmath>
-
-namespace Catch {
-
-#if !defined(CATCH_CONFIG_POLYFILL_ISNAN)
-    bool isnan(float f) {
-        return std::isnan(f);
-    }
-    bool isnan(double d) {
-        return std::isnan(d);
-    }
-#else
-    // For now we only use this for embarcadero
-    bool isnan(float f) {
-        return std::_isnan(f);
-    }
-    bool isnan(double d) {
-        return std::_isnan(d);
-    }
-#endif
-
-} // end namespace Catch