libcxx: Define set_unexpected, _get_unexpected and __uncaught_exceptions without dllimport.

It turns out that the MSVC headers define these functions without
dllimport even when compiling with /MD. This change fixes the resulting
compile-time error.

Differential Revision: https://reviews.llvm.org/D42207

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@322794 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed