[cxx-interop] Do not crash when emitting layout of std::string

If a `[[no_unique_address]]` field has zero size according to Clang, and field has a type that isn't representable in Swift, Swift would previously try to add an opaque field of size 1 for it.

This is wrong and was causing crashes for `std::string` while emitting a padding field:
```
_LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);
```

rdar://151941799

Author: egorzhdan

lib/IRGen/GenStruct.cpp

@@ -1480,9 +1480,11 @@ class ClangRecordLowering {
     }
 
     // Otherwise, add it as an opaque blob.
-    auto fieldSize = isZeroSized ? clang::CharUnits::Zero() : ClangContext.getTypeSizeInChars(clangField->getType());
-    return addOpaqueField(offset,
-                          dataSize.value_or(Size(fieldSize.getQuantity())));
+    auto fieldTypeSize = ClangContext.getTypeSizeInChars(clangField->getType());
+    auto fieldSize = isZeroSized
+                         ? clang::CharUnits::Zero()
+                         : dataSize.value_or(Size(fieldTypeSize.getQuantity()));
+    return addOpaqueField(offset, Size(fieldSize.getQuantity()));
   }
 
   /// Add opaque storage for bitfields spanning the given range of bits.