[MinGW] Fix dllexport of explicit template instantiation

Contrary to MSVC, GCC/MinGW needs to have the dllexport attribute
on the template instantiation declaration, not on the definition.

Previously clang never marked explicit template instantiations as
dllexport in MinGW mode, if the instantiation had a previous
declaration, regardless of where the attribute was placed. This
makes Clang behave like GCC in this regard, and allows using the
same attribute form for both MinGW compilers.

This fixes PR40256.

Differential Revision: https://reviews.llvm.org/D61118

llvm-svn: 359285