[SandboxIR] Fix warning when building on Windows with clang-cl. NFC.

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
      |                                                    ^
```

Author: aganea

llvm/lib/SandboxIR/Constant.cpp

@@ -324,7 +324,7 @@ template class GlobalWithNodeAPI<GlobalVariable, llvm::GlobalVariable,
 template class GlobalWithNodeAPI<GlobalAlias, llvm::GlobalAlias, GlobalValue,
                                  llvm::GlobalValue>;
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 // These are needed for SandboxIRTest when building with LLVM_BUILD_LLVM_DYLIB
 template LLVM_EXPORT_TEMPLATE GlobalIFunc &
 GlobalWithNodeAPI<GlobalIFunc, llvm::GlobalIFunc, GlobalObject,