[LifetimeSafety] Handle pruned-edges (null blocks) in dataflow (#150670)

Fix a crash in the lifetime safety dataflow analysis when handling null CFG blocks.

Added a null check for adjacent blocks in the dataflow analysis algorithm to prevent dereferencing null pointers. This occurs when processing CFG blocks with unreachable successors or predecessors.

Original crash: https://compiler-explorer.com/z/qfzfqG5vM

Fixes #150095

Author: usx95

clang/lib/Analysis/LifetimeSafety.cpp

@@ -603,6 +603,8 @@ class DataflowAnalysis {
       OutStates[B] = StateOut;
       Visited.set(B->getBlockID());
       for (const CFGBlock *AdjacentB : isForward() ? B->succs() : B->preds()) {
+        if (!AdjacentB)
+          continue;
         Lattice OldInState = getInState(AdjacentB);
         Lattice NewInState = D.join(OldInState, StateOut);
         // Enqueue the adjacent block if its in-state has changed or if we have

clang/unittests/Analysis/LifetimeSafetyTest.cpp

@@ -512,6 +512,25 @@ TEST_F(LifetimeAnalysisTest, PointersAndExpirationInACycle) {
   EXPECT_THAT(LoansTo({"temp"}), AreExpiredAt("after_loop"));
 }
 
+TEST_F(LifetimeAnalysisTest, InfiniteLoopPrunesEdges) {
+  SetupTest(R"(
+    void target(MyObj out) {
+      MyObj *p = &out;
+      POINT(before_loop);
+
+      for (;;) {
+        POINT(begin);
+        MyObj in;
+        p = ∈
+        POINT(end);
+      }
+    }
+  )");
+  EXPECT_THAT(Origin("p"), HasLoansTo({"out"}, "before_loop"));
+  EXPECT_THAT(Origin("p"), HasLoansTo({"in", "out"}, "begin"));
+  EXPECT_THAT(Origin("p"), HasLoansTo({"in"}, "end"));
+}
+
 TEST_F(LifetimeAnalysisTest, NestedScopes) {
   SetupTest(R"(
     void target() {