[COFF] Warn for locally imported symbols

Locally imported symbols are a very surprising linker feature. link.exe
warns for them, and we should warn too.

Differential Revision: https://reviews.llvm.org/D41269

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320792 91177308-0d34-0410-b5e6-96231b3b80d8