[lld][WebAssembly] Fix for string merging + negative addends

[sbc100]

Summary

Don't include the relocation addend when calculating the
virtual address of a symbol. Instead just pass the symbol's
offset and add the addend afterwards.

Without this fix we hit the offset is outside the section
error in MergeInputSegment::getSegmentPiece.

This fixes a real world error we were are seeing in emscripten.

[sbc100]

rebase

lld/test/wasm/merge-string.s

 // RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
-// RUN: wasm-ld -O2 %t.o -o %t.wasm --no-gc-sections --no-entry
+// RUN: wasm-ld -O1 %t.o -o %t.wasm --no-gc-sections --no-entry
 // RUN: obj2yaml %t.wasm | FileCheck %s --check-prefixes=COMMON,MERGE
+
+// Check that the default is the same as -O1 (since we default to -O1)
+// RUN: wasm-ld %t.o -o %t.wasm --no-gc-sections --no-entry
+// RUN: obj2yaml %t.wasm | FileCheck %s --check-prefixes=COMMON,MERGE
+
+// Check that -O0 disables merging
 // RUN: wasm-ld -O0 %t.o -o %t2.wasm --no-gc-sections --no-entry
 // RUN: obj2yaml %t2.wasm | FileCheck --check-prefixes=COMMON,NOMERGE %s
 
         .section .rodata1,"S",@
         .asciz "abc"
 foo:
         .ascii "a"
         .size foo, 1
 bar:
         .asciz "bc"
         .asciz "bc"
         .size bar, 4
 
+        .section .rodata_relocs,"",@
+negative_addend:
+        .int32 foo-10
+        .size negative_addend, 4
+
 .globl foo
 .globl bar
 .export_name    foo, foo
 .export_name    bar, bar
 
 //       COMMON:  - Type:            GLOBAL
 //  COMMON-NEXT:    Globals:
 //  COMMON-NEXT:      - Index:           0

lld/wasm/InputFiles.cpp

case R_WASM_MEMORY_ADDR_LOCREL_I32: {
     // Treat non-TLS relocation against symbols that live in the TLS segment
     // like TLS relocations.  This beaviour exists to support older object
     // files created before we introduced TLS relocations.
     // TODO(sbc): Remove this legacy behaviour one day.  This will break
     // backward compat with old object files built with `-fPIC`.
     if (D->segment && D->segment->outputSeg->isTLS())
       return D->getOutputSegmentOffset() + reloc.Addend;
 
-    uint64_t value = D->getVA(reloc.Addend);
+    uint64_t value = D->getVA() + reloc.Addend;
     if (reloc.Type == R_WASM_MEMORY_ADDR_LOCREL_I32) {
       const auto *segment = cast<InputSegment>(chunk);
       uint64_t p = segment->outputSeg->startVA + segment->outputSegmentOffset +
                    reloc.Offset - segment->getInputSectionOffset();
       value -= p;
     }
     return value;
   }

lld/wasm/Symbols.h

public:
 
   // Constructor for linker synthetic data symbols.
   DefinedData(StringRef name, uint32_t flags)
       : DataSymbol(name, DefinedDataKind, flags, nullptr) {}
 
   static bool classof(const Symbol *s) { return s->kind() == DefinedDataKind; }
 
   // Returns the output virtual address of a defined data symbol.
-  uint64_t getVA(uint64_t addend = 0) const;
+  uint64_t getVA() const;
   void setVA(uint64_t va);
 
   // Returns the offset of a defined data symbol within its OutputSegment.
   uint64_t getOutputSegmentOffset() const;
   uint64_t getOutputSegmentIndex() const;
   uint64_t getSize() const { return size; }
 
   InputSegment *segment = nullptr;

lld/wasm/Symbols.cpp

 }
 
 DefinedFunction::DefinedFunction(StringRef name, uint32_t flags, InputFile *f,
                                  InputFunction *function)
     : FunctionSymbol(name, DefinedFunctionKind, flags, f,
                      function ? &function->signature : nullptr),
       function(function) {}
 
-uint64_t DefinedData::getVA(uint64_t addend) const {
+uint64_t DefinedData::getVA() const {
   LLVM_DEBUG(dbgs() << "getVA: " << getName() << "\n");
   if (segment)
-    return segment->getVA(value + addend);
+    return segment->getVA(value);
   return value;
 }
 
 void DefinedData::setVA(uint64_t value_) {
   LLVM_DEBUG(dbgs() << "setVA " << name << " -> " << value_ << "\n");
   assert(!segment);
   value = value_;
 }
 
 uint64_t DefinedData::getOutputSegmentOffset() const {
   LLVM_DEBUG(dbgs() << "getOutputSegmentOffset: " << getName() << "\n");
-  return segment->outputSegmentOffset + value;
+  return segment->getOffset(value);
 }
 
 uint64_t DefinedData::getOutputSegmentIndex() const {
   LLVM_DEBUG(dbgs() << "getOutputSegmentIndex: " << getName() << "\n");
   return segment->outputSeg->index;
 }
 
 uint32_t GlobalSymbol::getGlobalIndex() const {