Implement lldb formatter for "clang encoded" enums (LLDB 18.1+) (V3)
This is a redo of PR (#124458) which was approved previously but force-pushed out. Then a V2 (#124745) failed `debuginfo\msvc-pretty-enums.rs` test during merge.
I've fixed the test and checked it to pass on Windows with `.\x.ps1 test .\tests\debuginfo\msvc-pretty-enums.rs`
Below is the original summary:
## Summary:
fixes#79530
I landed a fix last year to enable `DW_TAG_variant_part` encoding in LLDBs (https://reviews.llvm.org/D149213). This PR is a corresponding fix in synthetic formatters to decode that information.
This is in no way perfect implementation but at least it improves the status quo. But most types of enums will be visible and debuggable in some way.
I've also updated most of the existing tests that touch enums and re-enabled test cases based on LLDB for enums.
## Test Plan:
ran tests `./x test tests/debuginfo/`. Also tested manually in LLDB CLI and LLDB VSCode
## Other Thoughs:
A better approach would probably be adopting [formatters from codelldb](https://github.com/vadimcn/codelldb/blob/master/formatters/rust.py). There is some neat hack that hooks up summary provider via synthetic provider which can ultimately fix more display issues for Rust types and enums too. But getting it to work well might take more time that I have right now.
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