Allow empty mappings for optional YAML input

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.

Reviewers: compnerd, jakehehrlich, silvas, kledzik, mehdi_amini, pcc

Reviewed By: compnerd

Subscribers: silvas, llvm-commits

Differential Revision: https://reviews.llvm.org/D39908

llvm-svn: 318428

lib/Support/YAMLTraits.cpp

@@ -160,7 +160,8 @@ bool Input::preflightKey(const char *Key, bool Required, bool, bool &UseDefault,
 
   MapHNode *MN = dyn_cast<MapHNode>(CurrentNode);
   if (!MN) {
-    setError(CurrentNode, "not a mapping");
+    if (Required || !isa<EmptyHNode>(CurrentNode))
+      setError(CurrentNode, "not a mapping");
     return false;
   }
   MN->ValidKeys.push_back(Key);

test/tools/yaml2obj/empty-symbols.yaml

@@ -0,0 +1,10 @@
+# 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: