[clang] [Clang] Apply exclude_from_explicit_instantiation to dllimport/dllexport (PR #168171)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem commented:

Sorry, this got lost in the email pile over the holidays.

I'm still confused about the change to `Sema::DefineUsedVTables`. I'd rather we just check whether *any* method has `ExcludeFromExplicitInstantiationAttr`.

https://github.com/llvm/llvm-project/pull/168171