[WebAssembly] MC: Use zero for provisional value of undefined symbols

[sbc100]

Summary

This is more in line with what happens in the final
exectuable when symbols are undefined (i.e. weak
references).

lib/MC/WasmObjectWriter.cpp

 // Compute a value to write into the code at the location covered
 // by RelEntry. This value isn't used by the static linker, since
 // we have addends; it just serves to make the code more readable
 // and to make standalone wasm modules directly usable.
 uint32_t
 WasmObjectWriter::getProvisionalValue(const WasmRelocationEntry &RelEntry) {
   const MCSymbolWasm *Sym = ResolveSymbol(*RelEntry.Symbol);
 
-  // For undefined symbols, use a hopefully invalid value.
+  // For undefined symbols, use zero
   if (!Sym->isDefined(/*SetUsed=*/false))
-    return UINT32_MAX;
+    return 0;
 
   uint32_t GlobalIndex = SymbolIndices[Sym];
   const WasmGlobal& Global = Globals[GlobalIndex - NumGlobalImports];
   uint64_t Address = Global.InitialValue + RelEntry.Addend;
 
   // Ignore overflow. LLVM allows address arithmetic to silently wrap.
   uint32_t Value = Address;
 

test/MC/WebAssembly/external-data.ll

 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           0
 ; CHECK-NEXT:         Content:         '0700000000000000'
 ; CHECK-NEXT:       - SectionOffset:   19
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           8
-; CHECK-NEXT:         Content:         FFFFFFFF
+; CHECK-NEXT:         Content:         '00000000'

test/MC/WebAssembly/init-fini-array.ll

 ; CHECK-NEXT:         Offset:          0x0000003F
 ; CHECK-NEXT:       - Type:            R_WEBASSEMBLY_FUNCTION_INDEX_LEB
 ; CHECK-NEXT:         Index:           1
 ; CHECK-NEXT:         Offset:          0x00000045
 ; CHECK-NEXT:     Functions:       
 ; CHECK-NEXT:       - Locals:          
 ; CHECK-NEXT:         Body:            1080808080000B
 ; CHECK-NEXT:       - Locals:          
-; CHECK-NEXT:         Body:            0240418080808000410041FFFFFFFF7F1081808080000D000F0B00000B
+; CHECK-NEXT:         Body:            024041808080800041004180808080001081808080000D000F0B00000B
 ; CHECK-NEXT:       - Locals:          
 ; CHECK-NEXT:         Body:            1082808080000B
 ; CHECK-NEXT:       - Locals:          
-; CHECK-NEXT:         Body:            0240418180808000410041FFFFFFFF7F1081808080000D000F0B00000B
+; CHECK-NEXT:         Body:            024041818080800041004180808080001081808080000D000F0B00000B
 ; CHECK-NEXT:   - Type:            DATA
 ; CHECK-NEXT:     Segments:        
 ; CHECK-NEXT:       - SectionOffset:   6
 ; CHECK-NEXT:         MemoryIndex:     0
 ; CHECK-NEXT:         Offset:          
 ; CHECK-NEXT:           Opcode:          I32_CONST
 ; CHECK-NEXT:           Value:           0
 ; CHECK-NEXT:         Content:         '01040000'