[lld][WebAssembly] Fix for string tail merging and -r/--relocatable

[sbc100]

Summary

Ensure that both SyntheticMergedChunk and all MergeInfoChunks that it
comprises are assigned the correct output section. Without this we
would crash when outputting relocations in --relocatable mode.

Fixes: https://github.com/emscripten-core/emscripten/issues/14220

[sbc100]

• update comment

lld/test/wasm/merge-string-debug.s

 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
 # RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/merge-string-debug2.s -o %t2.o
 
 # RUN: wasm-ld %t.o %t2.o -o %t.wasm --no-entry
 # RUN: llvm-readobj -x .debug_str %t.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O1
 
+# Check that we -r/--reclocatable can handle string merging too
+# RUN: wasm-ld --relocatable %t.o %t2.o -o %t3.o
+# RUN: wasm-ld -O1 %t3.o -o %t.wasm --no-entry
+# RUN: llvm-readobj -x .debug_str %t.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O1
+
 # RUN: wasm-ld -O0 %t.o %t2.o -o %tO0.wasm --no-entry
 # RUN: llvm-readobj -x .debug_str %tO0.wasm | FileCheck %s --check-prefixes CHECK,CHECK-O0
 
 .section .debug_str,"S",@
+.Linfo_string0:
   .asciz "clang version 13.0.0"
+.Linfo_string1:
   .asciz "foobar"
 
+.section .debug_other,"",@
+  .int32 .Linfo_string0
+
 # CHECK: Hex dump of section '.debug_str':
 
 # CHECK-O0: 0x00000000 636c616e 67207665 7273696f 6e203133 clang version 13
 # CHECK-O0: 0x00000010 2e302e30 00666f6f 62617200 636c616e .0.0.foobar.clan
 # CHECK-O0: 0x00000020 67207665 7273696f 6e203133 2e302e30 g version 13.0.0
 # CHECK-O0: 0x00000030 00626172 00666f6f 00                .bar.foo.
 
 # CHECK-O1: 0x00000000 666f6f62 61720066 6f6f0063 6c616e67 foobar.foo.clang
 # CHECK-O1: 0x00000010 20766572 73696f6e 2031332e 302e3000  version 13.0.0.

lld/wasm/OutputSections.cpp

 }
 
 // Lots of duplication here with OutputSegment::finalizeInputSegments
 void CustomSection::finalizeInputSections() {
   SyntheticMergedChunk *mergedSection = nullptr;
   std::vector<InputChunk *> newSections;
 
   for (InputChunk *s : inputSections) {
+    s->outputSec = this;
     MergeInputChunk *ms = dyn_cast<MergeInputChunk>(s);
     if (!ms) {
       newSections.push_back(s);
       continue;
     }
 
     if (!mergedSection) {
       mergedSection =
           make<SyntheticMergedChunk>(name, 0, WASM_SEG_FLAG_STRINGS);
       newSections.push_back(mergedSection);
+      mergedSection->outputSec = this;
     }
     mergedSection->addMergeChunk(ms);
   }
 
   if (!mergedSection)
     return;
 
   mergedSection->finalizeContents();
   inputSections = newSections;
 }
 
 void CustomSection::finalizeContents() {
   finalizeInputSections();
 
   raw_string_ostream os(nameData);
   encodeULEB128(name.size(), os);
   os << name;
   os.flush();
 
   for (InputChunk *section : inputSections) {
     assert(!section->discarded);
-    section->outputSec = this;
     section->outSecOff = payloadSize;
     payloadSize += section->getSize();
   }
 
   createHeader(payloadSize + nameData.size());
 }
 
 void CustomSection::writeTo(uint8_t *buf) {