[llvm] XCOFF associated metadata (PR #159096)

[Wael Yehia via llvm-commits]

================
@@ -766,6 +766,30 @@ void Verifier::visitGlobalValue(const GlobalValue &GV) {
                               DL.getIntPtrType(GO->getType()),
                               RangeLikeMetadataKind::AbsoluteSymbol);
     }
+
+    if (GO->hasMetadata(LLVMContext::MD_ref)) {
+      SmallVector<MDNode *> MDs;
+      GO->getMetadata(LLVMContext::MD_ref, MDs);
+      for (const MDNode *MD : MDs) {
+        Check(MD->getNumOperands() == 1, "ref metadata must have one operand",
----------------
w2yehia wrote:

since we're allowed to have multiple `!ref`, it's more compact to allow one `!ref` that references multiple globals? 
```
@a = global i32 3, !ref !1
!1 = !{ptr @b, ptr @c}
```
rather than

```
@a = global i32 3, !ref !1, !ref !2
!1 = !{ptr @b}
!2 = !{ptr @c}
```

https://llvm.org/docs/LangRef.html#callees-metadata does that.

https://github.com/llvm/llvm-project/pull/159096