[DWARFYAML] Make the length field of compilation units optional. NFC.

This patch makes the length field of compilation units optional (0 by
default).

Author: higuoxing

lldb/unittests/Expression/DWARFExpressionTest.cpp

@@ -94,8 +94,7 @@ TEST(DWARFExpression, DW_OP_convert) {
       "      - Attribute:       DW_AT_byte_size\n"
       "        Form:            DW_FORM_data1\n"
       "debug_info:\n"
-      "  - Length:          0\n"
-      "    Version:         4\n"
+      "  - Version:         4\n"
       "    AbbrOffset:      0\n"
       "    AddrSize:        8\n"
       "    Entries:\n"

lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp

@@ -56,8 +56,7 @@ TEST_F(DWARFASTParserClangTests,
       "      - Attribute:       DW_AT_byte_size\n"
       "        Form:            DW_FORM_data1\n"
       "debug_info:\n"
-      "  - Length:          0\n"
-      "    Version:         4\n"
+      "  - Version:         4\n"
       "    AbbrOffset:      0\n"
       "    AddrSize:        8\n"
       "    Entries:\n"

llvm/lib/ObjectYAML/DWARFYAML.cpp

@@ -143,7 +143,7 @@ void MappingTraits<DWARFYAML::PubSection>::mapping(
 
 void MappingTraits<DWARFYAML::Unit>::mapping(IO &IO, DWARFYAML::Unit &Unit) {
   IO.mapOptional("Format", Unit.Format, dwarf::DWARF32);
-  IO.mapRequired("Length", Unit.Length);
+  IO.mapOptional("Length", Unit.Length, 0);
   IO.mapRequired("Version", Unit.Version);
   if (Unit.Version >= 5)
     IO.mapRequired("UnitType", Unit.Type);

llvm/unittests/DebugInfo/DWARF/DWARFDebugInfoTest.cpp

@@ -1368,8 +1368,7 @@ TEST(DWARFDebugInfo, TestEmptyChildren) {
                          "    Children:        DW_CHILDREN_yes\n"
                          "    Attributes:\n"
                          "debug_info:\n"
-                         "  - Length:          0\n"
-                         "    Version:         4\n"
+                         "  - Version:         4\n"
                          "    AbbrOffset:      0\n"
                          "    AddrSize:        8\n"
                          "    Entries:\n"

llvm/unittests/DebugInfo/DWARF/DWARFDieTest.cpp

@@ -34,8 +34,7 @@ TEST(DWARFDie, getLocations) {
           - Attribute:       DW_AT_call_data_location
             Form:            DW_FORM_sec_offset
     debug_info:
-      - Length:          0
-        Version:         5
+      - Version:         5
         UnitType:        DW_UT_compile
         AbbrOffset:      0
         AddrSize:        4