MC: Diagnose an unexpected token in COFF .section instead of asserting

This can easily arise when trying to assemble and ELF style .section
directive for a COFF object file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215373 91177308-0d34-0410-b5e6-96231b3b80d8
Add missing test for r215031

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215374 91177308-0d34-0410-b5e6-96231b3b80d8