This fixes several of these:
```
[1151/3381] Building CXX object lib\SandboxIR\CMakeFiles\LLVMSandboxIR.dir\Constant.cpp.obj
C:\git\llvm-project\llvm\lib\SandboxIR\Constant.cpp(331,52): warning: duplicate explicit instantiation of 'operator()' ignored as a Microsoft extension [-Wmicrosoft-template]
331 | llvm::GlobalObject>::LLVMGVToGV::operator()(llvm::GlobalIFunc
| ^
C:\git\llvm-project\llvm\include\llvm/SandboxIR/Constant.h(816,52): note: previous explicit instantiation is here
816 | llvm::GlobalObject>::LLVMGVToGV::operator()(llvm::GlobalIFunc
| ^
```