[WebAssembly] Add more test coverage for reloctions against section symbols

[sbc100]

Summary

The only known user of this relocation type and symbol type is
the debug info sections, but we were not testing the --relocatable
output path.

This change adds a minimal test case to cover relocations against
section symbols includes --relocatable output.

[sbc100]

• remove restriction

[yurydelendik]

Looks good. Thanks

lld/test/wasm/section-symbol-relocs.yaml

+# RUN: yaml2obj %s -o %t.o
+# RUN: llc -filetype=obj  %S/Inputs/custom.ll -o %t2.o
+# RUN: wasm-ld --no-entry -o - %t2.o %t.o | obj2yaml | FileCheck %s
+# RUN: wasm-ld -r --no-entry -o - %t2.o %t.o | obj2yaml | FileCheck %s -check-prefix=RELOC
+
+--- !WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            CUSTOM
+    Name:            green
+    Payload:         'AA0000000000000000'
+    Relocations:
+      - Type:            R_WASM_SECTION_OFFSET_I32
+        Index:           0
+        Offset:          0x00000001
+      - Type:            R_WASM_SECTION_OFFSET_I32
+        Index:           1
+        Offset:          0x00000005
+  - Type:            CUSTOM
+    Name:            red
+    Payload:         'BB0000000000000000'
+  - Type:            CUSTOM
+    Name:            linking
+    Version:         2
+    SymbolTable:
+      - Index:           0
+        Kind:            SECTION
+        Section:         0
+        Flags:           [ BINDING_LOCAL ]
+      - Index:           1
+        Kind:            SECTION
+        Section:         1
+        Flags:           [ BINDING_LOCAL ]
+...
+
+# CHECK:        Name:            green
+# CHECK-NEXT:   Payload:         626172717578AA0600000003000000
+# CHECK:        Name:            red
+# CHECK-NEXT:   Payload:         666F6FBB0000000000000000
+
+# RELOC:        Relocations:
+# RELOC-NEXT:       - Type:            R_WASM_SECTION_OFFSET_I32
+# RELOC-NEXT:         Index:           0
+# RELOC-NEXT:         Offset:          0x00000007
+# RELOC-NEXT:         Addend:          6
+# RELOC-NEXT:       - Type:            R_WASM_SECTION_OFFSET_I32
+# RELOC-NEXT:         Index:           1
+# RELOC-NEXT:         Offset:          0x0000000B
+# RELOC-NEXT:         Addend:          3
+# RELOC-NEXT:    Name:            green

llvm/lib/ObjectYAML/WasmYAML.cpp

void MappingTraits<WasmYAML::Comdat>::mapping(IO &IO,
   IO.mapRequired("Name", Comdat.Name);
   IO.mapRequired("Entries", Comdat.Entries);
 }
 
 void MappingTraits<WasmYAML::SymbolInfo>::mapping(IO &IO,
                                                   WasmYAML::SymbolInfo &Info) {
   IO.mapRequired("Index", Info.Index);
   IO.mapRequired("Kind", Info.Kind);
+  if (Info.Kind != wasm::WASM_SYMBOL_TYPE_SECTION)
     IO.mapRequired("Name", Info.Name);
   IO.mapRequired("Flags", Info.Flags);
   if (Info.Kind == wasm::WASM_SYMBOL_TYPE_FUNCTION) {
     IO.mapRequired("Function", Info.ElementIndex);
   } else if (Info.Kind == wasm::WASM_SYMBOL_TYPE_GLOBAL) {
     IO.mapRequired("Global", Info.ElementIndex);
   } else if (Info.Kind == wasm::WASM_SYMBOL_TYPE_EVENT) {
     IO.mapRequired("Event", Info.ElementIndex);
   } else if (Info.Kind == wasm::WASM_SYMBOL_TYPE_DATA) {

llvm/tools/yaml2obj/yaml2wasm.cpp

int WasmWriter::writeRelocSection(raw_ostream &OS, WasmYAML::Section &Sec,
   case wasm::WASM_SEC_CODE:
     writeStringRef("reloc.CODE", OS);
     break;
   case wasm::WASM_SEC_DATA:
     writeStringRef("reloc.DATA", OS);
     break;
   case wasm::WASM_SEC_CUSTOM: {
     auto CustomSection = dyn_cast<WasmYAML::CustomSection>(&Sec);
-    if (!CustomSection->Name.startswith(".debug_")) {
-      llvm_unreachable("not yet implemented (only for debug sections)");
-      return 1;
-    }
-
     writeStringRef(("reloc." + CustomSection->Name).str(), OS);
     break;
   }
   default:
     llvm_unreachable("not yet implemented");
     return 1;
   }