Modules: Code generation of enum constants for merged enum definitions

Found in a bootstrap of LLVM with implicit modules, resulting in a
deadlock of some Orc unit tests with libstdc++ 8.1. An enum was used as
part of the implementation of std::recursive_mutex and this bug resulted
in the constant initialization of zero instead of the desired non-zero
value. => Badness.

Richard Smith tells me neither of these fields are necessarily canonical
& so using declaresSamEntity is the right solution here (rather than
changing both of these Fields to be canonical by construction/from their
source)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361428 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGExprConstant.cpp

@@ -476,7 +476,7 @@ bool ConstStructBuilder::Build(const APValue &Val, const RecordDecl *RD,
   for (RecordDecl::field_iterator Field = RD->field_begin(),
        FieldEnd = RD->field_end(); Field != FieldEnd; ++Field, ++FieldNo) {
     // If this is a union, skip all the fields that aren't being initialized.
-    if (RD->isUnion() && Val.getUnionField() != *Field)
+    if (RD->isUnion() && !declaresSameEntity(Val.getUnionField(), *Field))
       continue;
 
     // Don't emit anonymous bitfields, they just affect layout.

test/Modules/enum-codegen.cpp

@@ -0,0 +1,36 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fmodules-cache-path=%t %s -emit-llvm -o - | FileCheck %s
+
+// CHECK: @_Z3varIiE = {{.*}} %union.union_type { i8 1 },
+
+#pragma clang module build bar
+module bar {
+  header "bar.h" { size 40 mtime 0 }
+  export *
+}
+#pragma clang module contents
+#pragma clang module begin bar
+union union_type {
+  char h{1};
+};
+#pragma clang module end
+#pragma clang module endbuild
+#pragma clang module build foo
+module foo {
+  header "foo.h" { size 97 mtime 0 }
+  export *
+}
+#pragma clang module contents
+#pragma clang module begin foo
+union union_type {
+  char h{1};
+};
+#pragma clang module import bar
+template<typename T>
+union_type var;
+#pragma clang module end
+#pragma clang module endbuild
+#pragma clang module import foo
+int main() {
+  (void)&var<int>;
+}