[ms][dll] #26935 Defining a dllimport function should cause it to be exported

If we have some function with dllimport attribute and then we have the function
definition in the same module but without dllimport attribute we should add
dllexport attribute to this function definition.
The same should be done for variables.

Example:
struct __declspec(dllimport) C3 {
  ~C3();
};
C3::~C3() {;} // we should export this definition.

Patch by Andrew V. Tischenko

Differential revision: http://reviews.llvm.org/D18953

llvm-svn: 270686