Allow empty mappings for optional YAML input

[kastiglione]

Summary

This change fixes a bug where obj2yaml can in some cases produce YAML that
causes yaml2obj to error.

The ELF YAML document structure has a Sections mapping, which contains three
mappings, all of which are optional: Local, Global, and Weak. Any one of
these can be missing, but if all three are missing, then yaml2obj errors. This
change allows YAML input for cases like this one.

I have tested this with check-llvm and check-lld, and all tests passed.

This change is the result of test failures while working on D39582, which
introduces a DynamicSymbols mapping, which will be empty at times.

[kastiglione]

Hi @silvas, this is a small change and related to the other ELF/YAML changes I made. Does this look ok to you?

[compnerd]

lib/Support/YAMLTraits.cpp

164

I would inline and apply Demorgan's:

if (Required || !isa<EmptyHNode>(CurrentNode))
  setError(...)

[kastiglione]

inline if condition

lib/Support/YAMLTraits.cpp

bool Input::preflightKey(const char *Key, bool Required, bool, bool &UseDefault,
   if (!CurrentNode) {
     if (Required)
       EC = make_error_code(errc::invalid_argument);
     return false;
   }
 
   MapHNode *MN = dyn_cast<MapHNode>(CurrentNode);
   if (!MN) {
+    bool OptionalNode = !Required && isa<EmptyHNode>(CurrentNode);
+    if (!OptionalNode)
       setError(CurrentNode, "not a mapping");
     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

test/tools/yaml2obj/empty-symbols.yaml

+# Ensure yaml2obj doesn't error on empty optional mappings, such as Symbols
+# RUN: yaml2obj %s -o %t
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_DYN
+  Machine:         EM_X86_64
+Symbols: