[SandboxIR] Silence warning with Clang 19.1.0 on Windows
This fixes:
```
[1832/3777] Building CXX object
lib\SandboxIR\CMakeFiles\LLVMSandboxIR.dir\SandboxIR.cpp.obj
In file included from
C:\src\git\llvm-project\llvm\lib\SandboxIR\SandboxIR.cpp:9:
C:\src\git\llvm-project\llvm\include\llvm/SandboxIR/SandboxIR.h(4400,16):
warning: unqualified friend declaration referring to type outside of the
nearest enclosing namespace is a Microsoft extension; add a nested name
specifier [-Wmicrosoft-unqualified-friend]
4400 | friend class TargetExtType; // For LLVMCtx.
| ^
| ::llvm::
1 warning generated.
```
