CodeGenCXX: no default dllimport storage for mingw

GNU frontends don't have options like /MT, /MD
This fixes a few link error regressions with libc++ and libc++abi

Reviewers: rnk, mstorsjo, compnerd

Differential Revision: https://reviews.llvm.org/D33620

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317398 91177308-0d34-0410-b5e6-96231b3b80d8