clang-cl: support __cdecl-on-struct anachronism

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263947 91177308-0d34-0410-b5e6-96231b3b80d8