Downgrade "definition of dllimport static field" error to warning for class templates...

Downgrade "definition of dllimport static field" error to warning for class templates (PR19902)

This allows us to compile the following kind of code, which occurs in MSVC
headers:

  template <typename> struct S {
    __declspec(dllimport) static int x;
  };
  template <typename T> int S<T>::x;

The definition works similarly to a dllimport inline function definition and
gets available_externally linkage.

Differential Revision: http://reviews.llvm.org/D3998

llvm-svn: 210141