[yaml2obj][MachO] - Fix PubName/PubType handling.

[grimar]

Summary

PubName and PubType are optional fields since D80722.

They are defined as:

Optional<PubSection> PubNames;
Optional<PubSection> PubTypes;

And initialized in the following way:

IO.mapOptional("debug_pubnames", DWARF.PubNames);
IO.mapOptional("debug_pubtypes", DWARF.PubTypes);

But problem is that because of the issue in YAMLTraits.cpp,
when there are no debug_pubnames/debug_pubtypes keys in a YAML description,
they are not initialized to Optional::None as the code expects, but they
are initialized to default PubSection() instances.

Because of this, the if condition in the following code is always true:

if (Obj.DWARF.PubNames)
  Err = DWARFYAML::emitPubSection(OS, *Obj.DWARF.PubNames,
                                  Obj.IsLittleEndian);

What means emitPubSection is always called and it writes few values.

This patch fixes the issue. I've reduced sizeofcmds by size of data
previously written because of this bug.

I need this fix (in YAMLTraits.cpp) for D81655.

[MaskRay]

Looks great!

[Higuoxing]

Thanks!

llvm/lib/Support/YAMLTraits.cpp

if (Required)
       EC = make_error_code(errc::invalid_argument);
     return false;
   }
 
   MapHNode *MN = dyn_cast<MapHNode>(CurrentNode);
   if (!MN) {
     if (Required || !isa<EmptyHNode>(CurrentNode))
       setError(CurrentNode, "not a mapping");
+    else
+      UseDefault = true;
     return false;
   }
   MN->ValidKeys.push_back(Key);
   HNode *Value = MN->Mapping[Key].get();
   if (!Value) {
     if (Required)
       setError(CurrentNode, Twine("missing required key '") + Key + "'");
     else

llvm/test/ObjectYAML/MachO/DWARF-pubsections.yaml

 #CHECK:       - DieOffset:       0x00000071
 #CHECK:         Name:            char
 #CHECK: ...
 
 ## b) Test that yaml2obj will not emit the .debug_pubnames/.debug_pubtypes section's
 ## contents, if the "debug_pubnames"/"debug_pubtypes" entry doesn't exist in the
 ## "DWARF" entry.
 
-# RUN: yaml2obj --docnum=2 %s | obj2yaml | FileCheck %s --check-prefix=EMPTY
+# RUN: yaml2obj --docnum=2 %s -o %t2
+# RUN: obj2yaml %t2 | FileCheck %s --check-prefix=EMPTY
 
 #      EMPTY: Sections:
 # EMPTY-NEXT:   - sectname:        __debug_pubnames
 # EMPTY-NEXT:     segname:         __DWARF
 # EMPTY-NEXT:     addr:            0x0000000000000000
 # EMPTY-NEXT:     size:            0
 # EMPTY-NEXT:     offset:          0x00000000
 # EMPTY-NEXT:     align:           0

 
 --- !mach-o
 FileHeader:
   magic:      0xFEEDFACF
   cputype:    0x01000007
   cpusubtype: 0x00000003
   filetype:   0x0000000A
   ncmds:      1
-  sizeofcmds: 1800
+  sizeofcmds: 1772
   flags:      0x00000000
   reserved:   0x00000000
 LoadCommands:
   - cmd:      LC_SEGMENT_64
     cmdsize:  232
     segname:  __DWARF
     vmaddr:   0x00000000
     vmsize:   0x00000000