clang-cl: support __cdecl-on-struct anachronism

Summary:
The Microsoft compiler emits

  warning C4229: anachronism used : modifiers on data are ignored

for

  struct {} __cdecl s;

but ICU's gendict can generate such (and does when building
LibreOffice), so accepting this in clang-cl too would be useful.

Reviewers: rnk

Patch by Stephan Bergmann

Differential Revision: http://reviews.llvm.org/D16628

llvm-svn: 263947