[llvm][TableGen] Define FieldInit::isConcrete overload

Summary:
There are a few field init values that are concrete but not complete/foldable (e.g. `?`). This allows for using those values as initializers without erroring out.

Example:

```
class A {
  string value = ?;
}
class B<A impl> : A {
  let value = impl.value; // This currently emits an error.
  let value = ?;          // This doesn't emit an error.
}
```

Differential Revision: https://reviews.llvm.org/D74360

include/llvm/TableGen/Record.h

@@ -1295,6 +1295,7 @@ public:
   Init *resolveReferences(Resolver &R) const override;
   Init *Fold(Record *CurRec) const;
 
+  bool isConcrete() const override;
   std::string getAsString() const override {
     return Rec->getAsString() + "." + FieldName->getValue().str();
   }

lib/TableGen/Record.cpp

@@ -1778,6 +1778,14 @@ Init *FieldInit::Fold(Record *CurRec) const {
   return const_cast<FieldInit *>(this);
 }
 
+bool FieldInit::isConcrete() const {
+  if (DefInit *DI = dyn_cast<DefInit>(Rec)) {
+    Init *FieldVal = DI->getDef()->getValue(FieldName)->getValue();
+    return FieldVal->isConcrete();
+  }
+  return false;
+}
+
 static void ProfileCondOpInit(FoldingSetNodeID &ID,
                              ArrayRef<Init *> CondRange,
                              ArrayRef<Init *> ValRange,

test/TableGen/field-access-initializers.td

@@ -0,0 +1,23 @@
+// RUN: llvm-tblgen %s | FileCheck %s
+// XFAIL: vg_leak
+
+// CHECK: --- Defs ---
+
+// CHECK: def A1 {
+// CHECK:   string value = ?;
+// CHECK: }
+
+// CHECK: def B1 {
+// CHECK:   string value = A1.value;
+// CHECK: }
+
+class A {
+  string value = ?;
+}
+
+class B<A impl> : A {
+  let value = impl.value;
+}
+
+def A1 : A;
+def B1 : B<A1>;