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 w...