[DebugInfo] Emit a 1-byte value as a terminator of entries list in the name index.

As stated in section 6.1.1.2, DWARFv5, p. 142,
| The last entry for each name is followed by a zero byte that
| terminates the list. There may be gaps between the lists.

The patch changes emitting a 4-byte zero value to a 1-byte one, which
effectively removes the gap between entry lists, and thus saves
approximately 3 bytes per name; the calculation is not exact because
the total size of the table is aligned to 4.

Differential Revision: https://reviews.llvm.org/D86927

Author: igorkudrin

llvm/lib/CodeGen/AsmPrinter/AccelTable.cpp

@@ -505,7 +505,7 @@ template <typename DataT> void Dwarf5AccelTableWriter<DataT>::emitData() const {
       for (const auto *Value : Hash->Values)
         emitEntry(*static_cast<const DataT *>(Value));
       Asm->OutStreamer->AddComment("End of list: " + Hash->Name.getString());
-      Asm->emitInt32(0);
+      Asm->emitInt8(0);
     }
   }
 }

llvm/test/DebugInfo/X86/debug-names-end-of-list.ll

@@ -0,0 +1,24 @@
+; This checks that the list of index entries in the name index is terminated
+; with a 1-byte value.
+
+; RUN: llc -mtriple x86_64 -accel-tables=Dwarf -dwarf-version=5 -filetype=asm %s -o - | \
+; RUN:   FileCheck %s
+
+; CHECK:   .section .debug_names,"",@progbits
+; CHECK: .Lnames_entries0:
+; CHECK:   .byte 0    # End of list: int
+; CHECK:   .byte 0    # End of list: foo
+
+@foo = common dso_local global i32 0, align 4, !dbg !5
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !DIFile(filename: "foo.c", directory: "/tmp")
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "Manual", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !4)
+!3 = !{}
+!4 = !{!5}
+!5 = !DIGlobalVariableExpression(var: !6, expr: !DIExpression())
+!6 = distinct !DIGlobalVariable(name: "foo", scope: !2, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true)
+!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)