WebAssembly: properly handle MCBinaryExpr in aliased symbols

Swift uses aliased symbols to refer to offsets within metadata structures.

eg

```
@"$s12SwiftPrivate28_stdlib_ShardedAtomicCounterVN" = alias %swift.type, bitcast (i32* getelementptr inbounds (<{ i8**, i32, <{ i32, i32, i32, i32, i32, i32, i32 }>*, i32, i32 }>, <{ i8**, i32, <{ i32, i32, i32, i32, i32, i32, i32 }>*, i32, i32 }>* @"$s12SwiftPrivate28_stdlib_ShardedAtomicCounterVMf", i32 0, i32 1) to %swift.type*)
```

My previous commit only got the alias to refer to the correct target symbol,
but not at the right offset.

This commit properly emits these symbols.

With this change, print("hello world") works but print() with anything else is still broken.

Author: zhuowei

lib/MC/WasmObjectWriter.cpp

@@ -1199,6 +1199,23 @@ static bool isInSymtab(const MCSymbolWasm &Sym) {
   return true;
 }
 
+// SwiftWasm: takes a MCSymbolWasm that's an alias expression of the form
+// ((targetSymbol + constantA) - constantB) + constantC...)
+// return the final offset from targetSymbol.
+// if no offset, returns 0.
+static int64_t getAliasedSymbolOffset(const MCSymbolWasm &Symbol,
+                                      const MCAsmLayout &Layout) {
+  if (!Symbol.isVariable())
+    return 0;
+  const MCExpr *Expr = Symbol.getVariableValue();
+  MCValue Res;
+  if (!Expr->evaluateAsRelocatable(Res, &Layout, nullptr)) {
+    Expr->dump();
+    report_fatal_error("Can't evaluate alias symbol expression");
+  }
+  return Res.getConstant();
+}
+
 uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
                                        const MCAsmLayout &Layout) {
   uint64_t StartOffset = W.OS.tell();
@@ -1519,8 +1536,16 @@ uint64_t WasmObjectWriter::writeObject(MCAssembler &Asm,
       LLVM_DEBUG(dbgs() << "  -> index:" << WasmIndex << "\n");
     } else if (ResolvedSym->isData()) {
       assert(DataLocations.count(ResolvedSym) > 0);
-      const wasm::WasmDataReference &Ref =
+      // SwiftWasm: hack: grab the offset
+      // Swift has aliases of the form
+      // alias = ((symbol + constant) - constant)
+      // so we need to evaluate the constants here using MCExpr
+      // there's probably a proper way to do this.
+      int64_t Offset = getAliasedSymbolOffset(WS, Layout);
+      wasm::WasmDataReference Ref =
           DataLocations.find(ResolvedSym)->second;
+      Ref.Offset += Offset;
+      Ref.Size -= Offset;
       DataLocations[&WS] = Ref;
       LLVM_DEBUG(dbgs() << "  -> index:" << Ref.Segment << "\n");
     } else {