[Clang] Avoid using unwind library in the MSVC environment

We're seeing the following warnings with --rtlib=compiler-rt:

  lld-link: warning: ignoring unknown argument '--as-needed'
  lld-link: warning: ignoring unknown argument '-lunwind'
  lld-link: warning: ignoring unknown argument '--no-as-needed'

MSVC doesn't use the unwind library, so just omit it.

Differential Revision: https://reviews.llvm.org/D132440