[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.

test/Interop/Cxx/class/Inputs/member-variables.h

@@ -83,4 +83,21 @@ inline int takesZeroSizedInCpp(HasZeroSizedField x) {
   return x.a;
 }
 
+/// Not imported into Swift.
+struct EmptyNotImported {
+  char : 0;
+
+  EmptyNotImported() = default;
+  EmptyNotImported(const EmptyNotImported &) = delete;
+  EmptyNotImported(EmptyNotImported &&) = delete;
+};
+
+struct LastFieldNoUniqueAddress {
+  char c;
+  [[no_unique_address]] EmptyNotImported p0;
+
+  LastFieldNoUniqueAddress(const LastFieldNoUniqueAddress &other) {}
+  LastFieldNoUniqueAddress(LastFieldNoUniqueAddress &&other) {}
+};
+
 #endif

test/Interop/Cxx/class/zero-sized-field.swift

@@ -83,4 +83,14 @@ FieldsTestSuite.test("Non-standard-layout field padding in templated class reuse
   expectEqual(s2.get_c(), 6)
 }
 
+FieldsTestSuite.test("Last field is no unique address") {
+  var s = LastFieldNoUniqueAddress()
+
+  s.c = 5
+  expectEqual(s.c, 5)
+
+  s.c = 6
+  expectEqual(s.c, 6)
+}
+
 runAllTests()