Allow unity-dumping multiple .cc files, and running on g++, clang, or MSVC.
Allow unity-dumping multiple .cc files, and running on g++, clang, or MSVC.

For MSVC we use Rextester, which seems to be a fairly recent version
of MSVC (it supports C++14 but not C++17).

Quuxplusone/from-scratch,Quuxplusone/from-scratch,Quuxplusone/from-scratch