[SandboxIR][NFC] Remove some unnecessary friend declarations

Author: vporpo

llvm/include/llvm/SandboxIR/Context.h

@@ -25,12 +25,9 @@ class Context {
   LLVMContext &LLVMCtx;
   friend class Type;        // For LLVMCtx.
   friend class PointerType; // For LLVMCtx.
-  friend class CmpInst; // For LLVMCtx. TODO: cleanup when sandboxir::VectorType
-                        // is complete
-  friend class IntegerType;           // For LLVMCtx.
-  friend class StructType;            // For LLVMCtx.
-  friend class ::llvm::TargetExtType; // For LLVMCtx.
-  friend class Region;                // For LLVMCtx.
+  friend class IntegerType; // For LLVMCtx.
+  friend class StructType;  // For LLVMCtx.
+  friend class Region;      // For LLVMCtx.
 
   Tracker IRTracker;