[DebugInfo][RemoveDIs] Fix error from not tracking seen debug labels (#88718)

Fixes the reported errors on:
#87379

A previous patch updated the bitcode reading for debug
intrinsics/records to not perform the expensive debug info format
conversion from records to intrinsics in cases where no records were
present, but the patch did not actually track when debug labels had been
seen, resulting in errors when parsing bitcode where functions contained
debug label records but no other debug records. This patch fixes that
case and adds a test for it.

Author: SLTozer

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6454,6 +6454,7 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
     case bitc::FUNC_CODE_DEBUG_RECORD_LABEL: {
       // DbgLabelRecords are placed after the Instructions that they are
       // attached to.
+      SeenDebugRecord = true;
       Instruction *Inst = getLastInstruction();
       if (!Inst)
         return error("Invalid dbg record: missing instruction");

llvm/test/Bitcode/dbg-label-record-bc.ll

@@ -0,0 +1,50 @@
+;; Tests that we can parse and print a function containing a debug label record
+;; and no other debug record kinds.
+
+; RUN: llvm-as --write-experimental-debuginfo-iterators-to-bitcode=true %s -o - \
+; RUN: | opt -S | FileCheck %s --check-prefixes=CHECK,INTRINSIC
+
+; RUN: llvm-as --write-experimental-debuginfo-iterators-to-bitcode=true %s -o - \
+; RUN: | opt -S --preserve-input-debuginfo-format=true \
+; RUN: | FileCheck %s --check-prefixes=CHECK,RECORD
+
+source_filename = "bbi-94196.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: void @foo()
+; CHECK: bar:
+; INTRINSIC-NEXT: call void @llvm.dbg.label(metadata ![[LABEL:[0-9]+]]), !dbg ![[LOC:[0-9]+]]
+; RECORD-NEXT: #dbg_label(![[LABEL:[0-9]+]], ![[LOC:[0-9]+]])
+
+; CHECK-DAG: ![[LABEL]] = !DILabel({{.*}}name: "bar"
+; CHECK-DAG: ![[LOC]] = !DILocation(line: 5, column: 1
+
+define dso_local void @foo() !dbg !5 {
+entry:
+  br label %bar, !dbg !9
+
+bar:                                              ; preds = %entry
+  tail call void @llvm.dbg.label(metadata !10), !dbg !11
+  ret void, !dbg !12
+}
+
+declare void @llvm.dbg.label(metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+!llvm.ident = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, producer: "clang version 19.0.0git", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "<stdin>", directory: "/home/gbtozers/dev/llvm-project-ddd-textual-ir")
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !{i32 1, !"wchar_size", i32 4}
+!4 = !{!"clang version 19.0.0git"}
+!5 = distinct !DISubprogram(name: "foo", scope: !6, file: !6, line: 1, type: !7, scopeLine: 2, spFlags: DISPFlagDefinition, unit: !0)
+!6 = !DIFile(filename: "bbi-94196.c", directory: "/home/gbtozers/dev/llvm-project-ddd-textual-ir")
+!7 = !DISubroutineType(types: !8)
+!8 = !{null}
+!9 = !DILocation(line: 3, column: 3, scope: !5)
+!10 = !DILabel(scope: !5, name: "bar", file: !6, line: 5)
+!11 = !DILocation(line: 5, column: 1, scope: !5)
+!12 = !DILocation(line: 6, column: 3, scope: !5)