[MC] Switch static const to an enum to silence MSVC linker warnings

[MC] Switch static const to an enum to silence MSVC linker warnings

Integral class statics are handled oddly in MSVC, we don't need them in
this case, use an enum instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241945 91177308-0d34-0410-b5e6-96231b3b80d8