[yaml2obj] - Teach yaml2obj to produce SHT_GROUP section with a custom Info field.

[grimar]

Summary

It is useful to allow this because we would be able to replace at least one binary
object committed in LLD and replace it with the yaml2obj based test.

[jhenderson]

LGTM, with nit.

test/tools/yaml2obj/elf-comdat-broken2.yaml

1

Maybe worth calling this broken-info rather than broken2, to be more descriptive.

18

Nit: with a custom Info value

test/tools/yaml2obj/elf-comdat-broken2.yaml

+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj -sections %t | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:               ELFCLASS64
+  Data:                ELFDATA2LSB
+  Type:                ET_REL
+  Machine:             EM_X86_64
+Sections:
+  - Name:              .group
+    Type:              SHT_GROUP
+    Link:              .symtab
+    Info:              12345
+    Members:
+      - SectionOrType: GRP_COMDAT
+
+## Check we are able to produce SHT_GROUP section with a custom Info (12345).
+# CHECK:      Name: .group
+# CHECK-NEXT: Type: SHT_GROUP
+# CHECK-NEXT: Flags [
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size:
+# CHECK-NEXT: Link:
+# CHECK-NEXT: Info: 12345

tools/yaml2obj/yaml2elf.cpp

else if (auto S = dyn_cast<ELFYAML::RelocationSection>(Sec.get())) {
         }
       }
       SHeader.sh_info = Index;
 
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
     } else if (auto S = dyn_cast<ELFYAML::Group>(Sec.get())) {
       unsigned SymIdx;
-      if (SymN2I.lookup(S->Info, SymIdx)) {
+      if (SymN2I.lookup(S->Info, SymIdx) && !to_integer(S->Info, SymIdx)) {
         WithColor::error() << "Unknown symbol referenced: '" << S->Info
                            << "' at YAML section '" << S->Name << "'.\n";
         return false;
       }
       SHeader.sh_info = SymIdx;
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
     } else if (auto S = dyn_cast<ELFYAML::MipsABIFlags>(Sec.get())) {