Enable C++ modules for CLHEP

The C++ modules feature as described in https://clang.llvm.org/docs/Modules.html allow producing a binary header representation to avoid redundant header reparsing.

This feature is used in ROOT's dictionary system since ROOT v6.20: https://github.com/root-project/root/blob/master/README/README.CXXMODULES.md

CMSSW and other experiment migrate their dictionaries to use the provided by ROOT C++ modules support: https://github.com/cms-sw/cmssw/issues/15248

Dictionaries which transiently include clhep can be further optimized by building a separate module for CLHEP which this MR aims for.

The current patch introduces a module.modulemap file containing a mapping between a binary artifact (a module or a pcm file) and a set of header files. The C++ modules are more picky on translation unit encapsulation and thus require all headers which a translation unit uses to be included. In addition to the missing include we outline a few virtual destructors to avoid pollution of .o files with weak virtual tables.

This patch enables builds with modules via rootcling/genreflex/rootcint and enables compile-time module builds if configured like: cmake -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_CXX_FLAGS="-fmodules -Xclang -fmodules-local-submodule-visibility" ../clhep/

The module.modulemap file is easy to maintain as it globs for the well-behaved header files and enumerates already the ones needed special treatment.

Once merged we will backport this to cmssw.