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

[IR] 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@241958 91177308-0d34-0410-b5e6-96231b3b80d8