Use StringRefMemoryObject in llvm-mc. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221536 91177308-0d34-0410-b5e6-96231b3b80d8
Don't redeclare a pure virtual method.

I.E., there is no value is having

void foo() override = 0;

If it is override it is already present in a base class. Since it is pure,
some other class will have to implement it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221537 91177308-0d34-0410-b5e6-96231b3b80d8