Update the error handling of lib/Linker.

Instead of passing a std::string&, use the new diagnostic infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220608 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused variable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220610 91177308-0d34-0410-b5e6-96231b3b80d8
Allow the C API users to keep relying on the OutMessages parameter.

Should fix the Ocaml tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220611 91177308-0d34-0410-b5e6-96231b3b80d8