scary looking warnings in Calypso/ddmd/cpp/

[timotheecour]

potential sources of bugs; would be nice to enable -Werror (perhaps with -Wno-error=foo to avoid error-ing on stuff that's originating from upstream ldc code, cf https://stackoverflow.com/questions/15500143/clang-promoting-all-warnings-to-errors-except-for)

ddmd/cpp/cppmodule.cpp:1801:25: warning: delete called on non-final 'cpp::Module' that has virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
                        delete m;

Calypso/ddmd/cpp/calypso.cpp:1078:9: warning: binding dereferenced null pointer to reference has undefined behavior [-Wnull-dereference]
        *static_cast<clang::CompilerInstance*>(nullptr), pchHeader, pchFilename, std::move(OS), Buffer));

ddmd/cpp/cppexpression.cpp:126:13: note: add missing switch cases
    switch (Op)

Calypso/ddmd/cpp/cppmodule.cpp:1130:19: warning: variable 'tp' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
    else if (auto TempTemp =

// etc; plus a bunch of warning: unused variable 'TND'