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

This fixes:
```
[2295/3381] Building CXX object lib\Passes\CMakeFiles\LLVMPasses.dir\PassBuilder.cpp.obj
In file included from C:\git\llvm-project\llvm\lib\Passes\PassBuilder.cpp:368:
In file included from C:\git\llvm-project\llvm\include\llvm/Transforms/Vectorize/SandboxVectorizer/SandboxVectorizer.h:16:
C:\git\llvm-project\llvm\include\llvm/SandboxIR/Context.h(73,16): 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]
   73 |   friend class Region;            // For LLVMCtx.
      |                ^
      |                ::llvm::
1 warning generated.
```

Author: aganea

llvm/include/llvm/SandboxIR/Context.h

@@ -25,6 +25,7 @@ class Argument;
 class BBIterator;
 class Constant;
 class Module;
+class Region;
 class Value;
 class Use;