[lldb][Formatter] Get element type for unordered_maps from __hash_table::value_type (#144517)

#143501 changes usage of
`__hash_value_type` in libcxx to an empty tag type. This type will no
longer have a definition in DWARF. Currently the LLDB unordered_map
formatter deduces the map's `element_type` by looking at the `__cc_`
member of `__hash_value_type`. But that will no longer work because we
only have its forward declaration. Since what we're really after is the
type that `__hash_value_type` is wrapping, we can just look at the
`__hash_table::value_type` typedef. With
#143501 that will now point to
the `std::pair` element type (which used to be what we got from
`__cc_`).

TBD: need to double-check this works for older layouts. Quick glance at
the code makes me suspicious of cases like `unordered_map<std::pair<int,
int>, int>`

Author: Michael137

lldb/source/Plugins/Language/CPlusPlus/LibCxxUnorderedMap.cpp

@@ -99,14 +99,20 @@ static bool isUnorderedMap(ConstString type_name) {
 
 CompilerType lldb_private::formatters::LibcxxStdUnorderedMapSyntheticFrontEnd::
     GetElementType(CompilerType table_type) {
-  auto element_type = table_type.GetTypedefedType().GetTypeTemplateArgument(0);
+  auto element_type =
+      table_type.GetDirectNestedTypeWithName("value_type").GetTypedefedType();
+
+  // In newer unordered_map layouts, the std::pair element type isn't wrapped
+  // in any helper types. So return it directly.
+  if (isStdTemplate(element_type.GetTypeName(), "pair"))
+    return element_type;
 
   // This synthetic provider is used for both unordered_(multi)map and
-  // unordered_(multi)set. For unordered_map, the element type has an
-  // additional type layer, an internal struct (`__hash_value_type`)
-  // that wraps a std::pair. Peel away the internal wrapper type - whose
-  // structure is of no value to users, to expose the std::pair. This
-  // matches the structure returned by the std::map synthetic provider.
+  // unordered_(multi)set. For older unordered_map layouts, the element type has
+  // an additional type layer, an internal struct (`__hash_value_type`) that
+  // wraps a std::pair. Peel away the internal wrapper type - whose structure is
+  // of no value to users, to expose the std::pair. This matches the structure
+  // returned by the std::map synthetic provider.
   if (isUnorderedMap(
           m_backend.GetCompilerType().GetCanonicalType().GetTypeName())) {
     std::string name;