Improve diagnostic when failing to synthesize implicit member due to dllexport (PR22591)

This is only a problem in C++03 mode targeting MS ABI (MinGW doesn't
export inline methods, and C++11 marks these methods implicitly
deleted).

Since targeting the MS ABI in pre-C++11 mode is a rare configuration,
this will probably not get fixed, but we can at least have a better
error message.

llvm-svn: 230115