[Backtracing][Linux] Add Linux support to swift-backtrace. #66410

This also adds a function to demangle a symbol, and a way for the backtracing code to report warning messages to the same place as the main runtime. I'd like to rename the _swift_isThunkFunction() SPI also, but we can't do that until we've made the changes to the _Backtracing library, so we'll do that there instead. rdar://110261430

This was added to a later PR, but not to this one, though we need it here. rdar://110261430

This should have been disabled until apple#66338. rdar://110261430

Mike and Max made various helpful suggestions, so I've added and updated various comments and amended the code to cope with partial reads and writes. rdar://110261430

Moved the comment for `_swift_backtrace_demangle` into the header file instead of it being in the implementation. rdar://110261430

The Swift backtracer's frame pointer unwinder cannot work on Linux without this change, because the compiler omits the frame pointer from the function in libSwift_Backtracing that actually captures the stack. rdar://110260855

Using `SwiftShims` is undesirable - it creates all kinds of build issues, and means shipping the `_SwiftBacktracing.h` header in the SDK, which is not necessary. While we're doing this, add the necessary definitions for reading ELF and DWARF information. rdar://110261712

Use the new module structure rather the old SwiftShims header. This is much cleaner and lets us include operating system headers to get the relevant definitions where possible. Add code to support ELF and DWARF, including decompression using zlib, zstd and liblzma if those turn out to be required and available. rdar://110261712

This is for compatibility, so that I can split up the PRs. We'll remove it in the next PR. rdar://110261712

There's a chance that pipes might perform a partial read; we should handle that case. rdar://110261712

The `status` argument to the `_swift_backtrace_demangle()` function isn't especially useful, won't match behaviour on Windows, and we actually don't use it in the Swift code that calls this SPI. Remove it. rdar://110261712

`__cxa_demangle()` is a rather unusual API; one of its "features" is that the pointer you pass in must either be `nullptr`, in which case it will call `malloc()` itself, _or_ it has to be a pointer to a block of memory allocated with `malloc()`, because `__cxa_demangle()` may `realloc()` it for you. This seems to me to be something of a non-obvious footgun, so we never pass the caller's pointer through to `__cxa_demangle()`, which lets them decide how they want to allocate space. rdar://110261712

…ule. There's a separate declaration here because we can't include the `Backtrace.h` header from inside `modules/Runtime/Runtime.h`. rdar://110261712

The `_Backtracing` module has a number of private implementation only imports that aren't used outside of the module and that don't require any additional libraries (hence they aren't relevant to the outside world). `verify_all_overlays.py` needs to know about these when it does its test, because it loadas the module as the main module, which results in implementation only imports being required instead of ignored. rdar://110261712

We need a Linux specific `Target` implementation, and a couple of minor tweaks to make things build everywhere. rdar://110262673

This reverts commit 0d7c45d.

We don't need these here. They were a leftover from previous code. rdar://110262673

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[Backtracing][Linux] Add Linux support to swift-backtrace. #66410

[Backtracing][Linux] Add Linux support to swift-backtrace. #66410

Commits on Jun 7, 2023