[Clang] Prevent null pointer dereference in designated initializer check (#97220)

Summary:
This patch adds an assertion in clang::SemaObjC::BuildInstanceMessage()
to ensure getCurMethodDecl() returns a valid method declaration,
addressing a static analyzer finding.

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Differential Revision: https://phabricator.intern.facebook.com/D60250674

Author: smanna12

clang/lib/Sema/SemaExprObjC.cpp

@@ -3206,9 +3206,10 @@ ExprResult SemaObjC::BuildInstanceMessage(
     }
     if (!isDesignatedInitChain) {
       const ObjCMethodDecl *InitMethod = nullptr;
+      auto *CurMD = SemaRef.getCurMethodDecl();
+      assert(CurMD && "Current method declaration should not be null");
       bool isDesignated =
-          SemaRef.getCurMethodDecl()->isDesignatedInitializerForTheInterface(
-              &InitMethod);
+          CurMD->isDesignatedInitializerForTheInterface(&InitMethod);
       assert(isDesignated && InitMethod);
       (void)isDesignated;
       Diag(SelLoc, SuperLoc.isValid() ?