Use lowercase windows.h for mingw cross compilation.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211604 91177308-0d34-0410-b5e6-96231b3b80d8
Allow static_assert inside an anonymous union; fixes PR20021 as well as implements C++ Issue 1940.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211606 91177308-0d34-0410-b5e6-96231b3b80d8