[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;