Vassil Vassilev authored May 23, 2020 and Lynn Garren committed Nov 09, 2020

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.

Conflicts:
	Matrix/src/DiagMatrix.cc
	Matrix/src/GenMatrix.cc
	Matrix/src/Matrix.cc
	Matrix/src/SymMatrix.cc
	Matrix/src/Vector.cc