[MS ABI] Don't generates code for unreferenced inline definitions of library builtins

We should only consider declarations which were written, implicit
declarations shouldn't be considered.

This fixes PR24084.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241941 91177308-0d34-0410-b5e6-96231b3b80d8