[PassManager] Drop Function.h and Module.h includes (NFC) (#96892)

Replace them with forward declarations.

Author: nikic

llvm/include/llvm/IR/PassManager.h

@@ -42,8 +42,6 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/TinyPtrVector.h"
 #include "llvm/IR/Analysis.h"
-#include "llvm/IR/Function.h"
-#include "llvm/IR/Module.h"
 #include "llvm/IR/PassManagerInternal.h"
 #include "llvm/Support/TypeName.h"
 #include <cassert>
@@ -58,6 +56,9 @@
 
 namespace llvm {
 
+class Function;
+class Module;
+
 // Forward declare the analysis manager template.
 template <typename IRUnitT, typename... ExtraArgTs> class AnalysisManager;