Warn about dllexported explicit class template instantiation declarations (PR22035)

Warn about dllexported explicit class template instantiation declarations (PR22035)

Clang would previously become confused and crash here.

It does not make a lot of sense to export these, so warning seems appropriate.

MSVC will export some member functions for this kind of specializations, whereas
MinGW ignores the dllexport-edness. The latter behaviour seems better.

Differential Revision: http://reviews.llvm.org/D6984

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@226208 91177308-0d34-0410-b5e6-96231b3b80d8