[Clang] Allow simpler visibility annotations when targeting win32 and mingw (#133699) MinGW and Win32 disagree on where the `__declspec(dllexport)` should be placed on extern template instantiations. However, there doesn't fundamentally seem to be a problem with putting the annotation in both places. This patch adds a new diagnostic group and `-Wattribute-ignored` warnings about where the attribute is placed if the attribute is different on the declaration and definition. There is another new warning group `-Wdllexport-explicit-instantiation` that also diagnoses places where the attribute is technically ignored, even though the correct place is also annotated. This makes it possible to significantly simplify libc++'s visibility annotations (see #133704).
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