[clang] Fix ForStmt mustprogress handling

[atmnpatel]

Summary

D86841 had an error where for statements with no conditional were
required to make progress. This is not true, this patch removes that
line, and adds regression tests.

[jdoerfert]

LGTM

clang/lib/CodeGen/CGStmt.cpp

void CodeGenFunction::EmitForStmt(const ForStmt &S,
   JumpDest Continue = getJumpDestInCurrentScope("for.cond");
   llvm::BasicBlock *CondBlock = Continue.getBlock();
   EmitBlock(CondBlock);
 
   bool LoopMustProgress = false;
   Expr::EvalResult Result;
   if (LanguageRequiresProgress()) {
     if (!S.getCond()) {
-      LoopMustProgress = true;
       FnIsMustProgress = false;
     } else if (!S.getCond()->EvaluateAsInt(Result, getContext())) {
       LoopMustProgress = true;
     }
   }
 
   const SourceRange &R = S.getSourceRange();
   LoopStack.push(CondBlock, CGM.getContext(), CGM.getCodeGenOpts(), ForAttrs,

clang/test/CodeGen/attr-mustprogress-1.c

 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-attributes
 // RUN: %clang_cc1 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c11 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c18 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c2x -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 
 int a = 0;
 int b = 0;
 
 // CHECK: Function Attrs: noinline nounwind optnone
+// CHECK-LABEL: @f0(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    br label [[FOR_COND:%.*]]
+// CHECK:       for.cond:
+// CHECK-NOT:    br label [[FOR_COND]], !llvm.loop !{{.*}}
+//
+void f0() {
+  for (; ;) ;
+}
+
+// CHECK: Function Attrs: noinline nounwind optnone
 // CHECK-LABEL: @f1(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    br i1 true, label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
 // CHECK-NEXT:    br label [[FOR_COND]]
 // CHECK:       for.end:

clang/test/CodeGen/attr-mustprogress-1.cpp

 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --check-attributes
 // RUN: %clang_cc1 -std=c++11 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++14 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++17 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++20 -triple=x86_64-unknown-linux-gnu -S -emit-llvm %s -o - | FileCheck %s
 
 int a = 0;
 int b = 0;
 
 // CHECK: Function Attrs: noinline nounwind optnone
+// CHECK-LABEL: @_Z2f0v(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    br label [[FOR_COND:%.*]]
+// CHECK:       for.cond:
+// CHECK-NOT:    br label [[FOR_COND]], !llvm.loop !{{.*}}
+void f0() {
+  for (; ;) ;
+}
+
+// CHECK: Function Attrs: noinline nounwind optnone
 // CHECK-LABEL: @_Z2f1v(
 // CHECK-NEXT:  entry:
 // CHECK-NEXT:    br label [[FOR_COND:%.*]]
 // CHECK:       for.cond:
 // CHECK-NEXT:    br i1 true, label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
 // CHECK:       for.body:
 // CHECK-NEXT:    br label [[FOR_COND]]
 // CHECK:       for.end: