fix warning about noreturn in sanitizer_win's internal__exit()

Summary:
A previous fix used __assume(0), but not all compilers know that control will
not pass that. This patch uses a macro which works in more compilers.

Reviewers: rnk

Subscribers: kubabrecka

Differential Revision: https://reviews.llvm.org/D28268

llvm-svn: 291042