[WebAssembly] MC: Fix references to undefined externals in data section

[sbc100]

Summary

Undefined externals don't need to have a size or an offset.
This was broken by r303915. Added a test for this case.

This fixes the "Compile LLVM Torture (o)" step on the wasm
waterfall.

[jgravelle-google]

I don't know that much about MC but looks fine to me

lib/MC/WasmObjectWriter.cpp

if (WS.isFunction()) {
         // An import; the index was assigned above.
         Index = SymbolIndices.find(&WS)->second;
       }
 
       // If needed, prepare the function to be called indirectly.
       if (IsAddressTaken.count(&WS))
         TableElems.push_back(Index);
     } else {
+      if (WS.isTemporary() && !WS.getSize())
+        continue;
+
+      if (WS.isDefined(false)) {
         if (WS.getOffset() != 0)
-        report_fatal_error("data sections must contain one variable each");
+          report_fatal_error("data sections must contain one variable each: " +
+                             WS.getName());
         if (!WS.getSize())
-        report_fatal_error("data symbols must have a size set with .size");
+          report_fatal_error("data symbols must have a size set with .size: " +
+                             WS.getName());
 
         int64_t Size = 0;
         if (!WS.getSize()->evaluateAsAbsolute(Size, Layout))
           report_fatal_error(".size expression must be evaluatable");
 
-      if (WS.isDefined(false)) {
         MCSectionWasm &DataSection =
             static_cast<MCSectionWasm &>(WS.getSection());
 
         if (uint64_t(Size) != Layout.getSectionFileSize(&DataSection))
           report_fatal_error("data sections must contain at most one variable");
 
         DataBytes.resize(alignTo(DataBytes.size(), DataSection.getAlignment()));
 

lib/Target/WebAssembly/known_gcc_test_failures.txt

 pr38533.c wasm-o
 pr41239.c wasm-o
 pr43385.c wasm-o
 pr43560.c wasm-o
 pr45695.c wasm-o
 pr49279.c wasm-o
 pr49390.c wasm-o
 pr52286.c wasm-o
-
-# fatal error: error in backend: data symbols must have a size set with .size
-921110-1.c wasm-o

test/MC/WebAssembly/external-data.ll

+; RUN: llc -mtriple wasm32-unknown-unknown-wasm -filetype=obj %s -o - | obj2yaml | FileCheck %s
+; Verify relocations are correctly generated for addresses of externals
+; in the data section.
+
+declare i32 @f1(...)
+
+@foo = global i64 7, align 4
+@far = local_unnamed_addr global i32 (...)* @f1, align 4
+
+; CHECK:   - Type:            DATA
+; CHECK:     Relocations:
+; CHECK:       - Type:            R_WEBASSEMBLY_GLOBAL_ADDR_I32
+; CHECK:         Index:           0
+; CHECK:         Offset:          0x0000000E
+; CHECK:     Segments:
+; CHECK:       - Index:           0
+; CHECK:         Offset:
+; CHECK:           Opcode:          I32_CONST
+; CHECK:           Value:           0
+; CHECK:         Content:         0700000000000000FFFFFFFF
+