[clang][analysis] Fix flaky clang/test/Analysis/live-stmts.cpp test (2nd attempt) (llvm#127406)

In my previous attempt (llvm#126913) of fixing the flaky case was on a good
track when I used the begin locations as a stable ordering. However, I
forgot to consider the case when the begin locations are the same among
the Exprs.

In an `EXPENSIVE_CHECKS` build, arrays are randomly shuffled prior to
sorting them. This exposed the flaky behavior much more often basically
breaking the "stability" of the vector - as it should.
Because of this, I had to revert the previous fix attempt in llvm#127034.

To fix this, I use this time `Expr::getID` for a stable ID for an Expr.

Hopefully fixes llvm#126619
Hopefully fixes llvm#126804

(cherry picked from commit f378e52)

Author: steakhal

clang/lib/Analysis/LiveVariables.cpp

@@ -662,12 +662,19 @@ void LiveVariables::dumpExprLiveness(const SourceManager &M) {
 }
 
 void LiveVariablesImpl::dumpExprLiveness(const SourceManager &M) {
+  const ASTContext &Ctx = analysisContext.getASTContext();
+  auto ByIDs = [&Ctx](const Expr *L, const Expr *R) {
+    return L->getID(Ctx) < R->getID(Ctx);
+  };
+
   // Don't iterate over blockEndsToLiveness directly because it's not sorted.
   for (const CFGBlock *B : *analysisContext.getCFG()) {
-
     llvm::errs() << "\n[ B" << B->getBlockID()
                  << " (live expressions at block exit) ]\n";
-    for (const Expr *E : blocksEndToLiveness[B].liveExprs) {
+    std::vector<const Expr *> LiveExprs;
+    llvm::append_range(LiveExprs, blocksEndToLiveness[B].liveExprs);
+    llvm::sort(LiveExprs, ByIDs);
+    for (const Expr *E : LiveExprs) {
       llvm::errs() << "\n";
       E->dump();
     }

clang/test/Analysis/live-stmts.cpp

@@ -44,6 +44,8 @@ int testThatDumperWorks(int x, int y, int z) {
 // CHECK-NEXT: ImplicitCastExpr {{.*}} <IntegralToBoolean>
 // CHECK-NEXT: `-ImplicitCastExpr {{.*}} <LValueToRValue>
 // CHECK-NEXT:   `-DeclRefExpr {{.*}} 'x' 'int'
+// CHECK-EMPTY:
+// CHECK-EMPTY:
 // CHECK: [ B4 (live expressions at block exit) ]
 // CHECK-EMPTY:
 // CHECK-NEXT: DeclRefExpr {{.*}} 'y' 'int'