[lldb][Formatter] Get element type for unordered_maps from __hash_table::value_type

llvm#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 llvm#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

@@ -99,10 +99,10 @@ 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();
 
   // This synthetic provider is used for both unordered_(multi)map and
-  // unordered_(multi)set. For unordered_map, the element type has an
+  // 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