Report error if -export-dynamic is used with -r

The combination of the two flags doesn't make sense. And other linkers
seem to just ignore --export-dynamic if --relocatable is given, but
we probably should report it as an error to let users know that is
an invalid combination.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43552

Differential Revision: https://reviews.llvm.org/D68441

llvm-svn: 374022