[MC] Add llvm_unreachable to toString to fix compile time warning.

Without this change, GCC 7 raises the warning below:
        control reaches end of non-void function

Reviewers: sbc100, andreadb

Reviewed By: andreadb

Differential Revision: https://reviews.llvm.org/D46304


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331255 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed