[IRGen] Fix crash when using C union types with layout strings (#63983)

rdar://106056035

Author: drexin

lib/IRGen/GenStruct.cpp

@@ -403,6 +403,9 @@ namespace {
       std::vector<TypeLayoutEntry *> fields;
       for (auto &field : getFields()) {
         auto fieldTy = field.getType(IGM, T);
+        if (!fieldTy) {
+          return IGM.typeLayoutCache.getOrCreateTypeInfoBasedEntry(*this, T);
+        }
         fields.push_back(
             field.getTypeInfo().buildTypeLayoutEntry(IGM, fieldTy, useStructLayouts));
       }

test/IRGen/Inputs/union_type.h

@@ -0,0 +1,5 @@
+union TestUnion {
+    int x;
+    char y[80];
+    void* z;
+};

test/IRGen/layout_string_union_types.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend -enable-experimental-feature LayoutStringValueWitnesses -enable-type-layout -emit-ir -import-objc-header %S/Inputs/union_type.h %s
+
+struct UnionWrapper {
+    let x: TestUnion
+}