[ORC] Drop Comdat when discarding IR symbol

According to the IR verifier, "Declaration[s] may not be in a Comdat!"

This is a re-commit of 76b3f0b and
87d7838 with updates to the test:
 * Force emission of the extra-module, to trigger the bug after D138264,
   by providing a second symbol @g, and making the comdat nodeduplicate.
   (Technically only one is needed, but two should be safer.)
 * Name the comdat $f to avoid failure on Windows:
   LLVM ERROR: Associative COMDAT symbol 'c' does not exist.
 * Mark the test as UNSUPPORTED on macOS, MachO doesn't support COMDATs.

Differential Revision: https://reviews.llvm.org/D142443

(cherry picked from commit 50ca8b3)

llvm/lib/ExecutionEngine/Orc/Layer.cpp

@@ -125,6 +125,10 @@ void IRMaterializationUnit::discard(const JITDylib &JD,
   assert(!I->second->isDeclaration() &&
          "Discard should only apply to definitions");
   I->second->setLinkage(GlobalValue::AvailableExternallyLinkage);
+  // According to the IR verifier, "Declaration[s] may not be in a Comdat!"
+  // Remove it, if this is a GlobalObject.
+  if (auto *GO = dyn_cast<GlobalObject>(I->second))
+    GO->setComdat(nullptr);
   SymbolToDefinition.erase(I);
 }
 

llvm/test/ExecutionEngine/Orc/Inputs/weak-comdat-def.ll

@@ -0,0 +1,12 @@
+define i32 @g() {
+entry:
+  ret i32 0
+}
+
+$f = comdat nodeduplicate
+
+define i32 @f() comdat {
+entry:
+  %0 = call i32 @g()
+  ret i32 %0
+}

llvm/test/ExecutionEngine/Orc/weak-comdat.ll

@@ -0,0 +1,18 @@
+; RUN: lli -extra-module %p/Inputs/weak-comdat-def.ll %s
+; UNSUPPORTED: target={{.*}}-darwin{{.*}}
+
+declare i32 @g()
+
+$f = comdat nodeduplicate
+
+define weak i32 @f() comdat {
+entry:
+  %0 = call i32 @g()
+  ret i32 %0
+}
+
+define i32 @main() {
+entry:
+  %0 = call i32 @f()
+  ret i32 %0
+}