[lldb] Add SBType::FindDirectNestedType() function (#68705)

This patch adds a `SBType::FindDirectNestedType(name)` function which performs a non-recursive search in given class for a type with specified name. The intent is to perform a fast search in debug info, so that it can be used in formatters, and let them remain responsive.

This is driven by my work on formatters for Clang and LLVM types. In particular, by [`PointerIntPairInfo::MaskAndShiftConstants`](https://github.com/llvm/llvm-project/blob/cde9f9df79805a0850310870d6dcc64004292727/llvm/include/llvm/ADT/PointerIntPair.h#L174C16-L174C16), which is required to extract pointer and integer from `PointerIntPair`.

Related Discourse thread: https://discourse.llvm.org/t/traversing-member-types-of-a-type/72452