IWYU: Update for Debian 13 CI job

`include-what-you-use` diagnostics, in practice, are specific to
the environment's compiler and standard library.  Update includes
to satisfy IWYU for our CI job under Debian 13.  Some patterns:

* Types named in virtual `override` signatures no longer require
  includes since the overridden signature already names them.

* A function argument's type needs to be included even if its constructor
  is called only by implicit conversion.  For example, constructing a
  `std::function` from a lambda now requires `<functional>`.

* Some prior mysterious `<type_traits>` inclusions are no longer required.

Source/cmExportTryCompileFileGenerator.h

@@ -11,11 +11,9 @@
 
 #include "cmExportCMakeConfigGenerator.h"
 
-class cmFileSet;
 class cmGeneratorTarget;
 class cmGlobalGenerator;
 class cmMakefile;
-class cmTargetExport;
 
 class cmExportTryCompileFileGenerator : public cmExportCMakeConfigGenerator
 {