Fix #3916 - undefined symbols with -dllimport=all on Windows

[kinke]

Instantiated data symbols were previously never dllimported with -dllimport=all. So if the parent template instance wasn't codegen'd into the binary directly, it remained undefined.

For -dllimport=defaultLibsOnly, the 'solution' to this problem was to define-on-declare data symbols instantiated from druntime/Phobos templates, making sure each binary defines all such symbols it references.

In both cases, switch to an approach where we dllimport all instantiated data symbols (or druntime/Phobos symbols only), and
dllexport them whenever defining them (so that other object files or binaries can import them). This may lead to more 'importing
locally defined symbol' linker warnings, but may also lead to less duplicates and possibly 'proper' sharing of instantiated globals
across the whole process.

This is superfluous and skipped with -linkonce-templates, as that mode defines all referenced instantiated symbols in each binary anyway, and so has already been a workaround.

[kinke]

At work, we're using -dllimport=defaultLibsOnly, for a huge project consisting of ~140 dub packages/static libs. There are lots of 'importing locally defined symbol' linker warnings now, but I think they can be suppressed via /warning:<number>, need to test. And annoyingly, one symbol remains undefined - a std.variant.VariantN!16 init symbol referenced by its TypeInfo.