[ELFYAML] Fix handling SHT_NOBITS sections by obj2yaml/yaml2obj tools

SHT_NOBITS sections do not have content in an object file. Now yaml2obj
tool does not accept `Content` field for such sections, and obj2yaml
tool does not attempt to read the section content from a file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241350 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/ELFYAML.h

@@ -85,7 +85,13 @@ struct SectionOrType {
 };
 
 struct Section {
-  enum class SectionKind { Group, RawContent, Relocation, MipsABIFlags };
+  enum class SectionKind {
+    Group,
+    RawContent,
+    Relocation,
+    NoBits,
+    MipsABIFlags
+  };
   SectionKind Kind;
   StringRef Name;
   ELF_SHT Type;
@@ -106,6 +112,14 @@ struct RawContentSection : Section {
   }
 };
 
+struct NoBitsSection : Section {
+  llvm::yaml::Hex64 Size;
+  NoBitsSection() : Section(SectionKind::NoBits) {}
+  static bool classof(const Section *S) {
+    return S->Kind == SectionKind::NoBits;
+  }
+};
+
 struct Group : Section {
   // Members of a group contain a flag and a list of section indices
   // that are part of the group.