[lldb][test] TestDataFormatterLibcxxStringSimulator.py: fix padding for current layout (llvm#108362)

IIUC, the history of `std::string`'s `__short` structure in the
alternate ABI layout (as recorded by the simulator test) looks as
follows:
* First layout ( `SUBCLASS_PADDING` is defined):
```
     struct __short
     {
         value_type __data_[__min_cap];
        struct
            : __padding<value_type>
        {
            unsigned char __size_;
        };
     };
```
* Then:
```
     struct __short
     {
        value_type __data_[__min_cap];
        unsigned char __padding[sizeof(value_type) - 1];
        unsigned char __size_;
     };
```
* Then, post-`BITMASKS`:
```
     struct __short
     {
        value_type __data_[__min_cap];
        unsigned char __padding[sizeof(value_type) - 1];
        unsigned char __size_ : 7;
        unsigned char __is_long_ : 1;
     };
```

Which is the one that's [on
top-of-tree](https://github.com/llvm/llvm-project/blob/89c10e27d8b4d5f44998aad9abd2590d9f96c5df/libcxx/include/string#L854-L859).

But for `REVISION > 1`, `BITMASKS` is never set, so for those tests we
lose the `__padding` member.

This patch fixes this by splitting out the `SUBCLASS_PADDING` out of the
ifdef.

Drive-by:
* Also run expression evaluator on the string to provide is with some
extra coverage.

(cherry picked from commit 4ca8fb1)

Author: Michael137

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py

@@ -22,6 +22,9 @@ def _run_test(self, defines):
         self.expect_var_path("shortstring", summary='"short"')
         self.expect_var_path("longstring", summary='"I am a very long string"')
 
+        self.expect_expr("shortstring", result_summary='"short"')
+        self.expect_expr("longstring", result_summary='"I am a very long string"')
+
 
 for v in [None, "ALTERNATE_LAYOUT"]:
     for r in range(5):

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/main.cpp

@@ -71,19 +71,20 @@ template <class _CharT, class _Traits, class _Allocator> class basic_string {
 
   struct __short {
     value_type __data_[__min_cap];
-#ifdef BITMASKS
 #ifdef SUBCLASS_PADDING
     struct : __padding<value_type> {
       unsigned char __size_;
     };
-#else
+#else // !SUBCLASS_PADDING
+
     unsigned char __padding[sizeof(value_type) - 1];
+#ifdef BITMASKS
     unsigned char __size_;
-#endif
 #else // !BITMASKS
     unsigned char __size_ : 7;
     unsigned char __is_long_ : 1;
-#endif
+#endif // BITMASKS
+#endif // SUBCLASS_PADDING
   };
 
 #ifdef BITMASKS