Build fails with latest MSVC 2019 (v16.11) due to /WX

[emmenlau]

Description

I get the following error since switching to latest MSVC 2019 (v16.11):

cl : Command line warning D9025 : overriding '/W4' with '/W3'
C:\gitlabci\doctest\doctest\parts\doctest.cpp(2132): error C2220: the following warning is treated as an error
C:\gitlabci\doctest\doctest\parts\doctest.cpp(2132): warning C5245: 'doctest::`anonymous-namespace'::XmlWriter::ScopedElement::operator =': unreferenced function with internal linkage has been removed
C:\gitlabci\doctest\doctest\parts\doctest.cpp(2132): note: to simplify migration, consider the temporary use of /Wv:18 flag with the version of the compiler with which you used to build without warnings

Extra information

• doctest version: v2.4.6
• Operating System: windows 10 x64
• Compiler+version: MSVC 2019 (v16.11.2)

[emmenlau]

Closing as duplicate of #533