Convert MC command line option for fatal assembler warnings into a
proper flag.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216471 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r210342 and r210343, add test case for the crasher.
PR 20642.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216475 91177308-0d34-0410-b5e6-96231b3b80d8