This patch makes it possible to detect in LLDB shell and API tests if
`-fbounds-safety` is supported by the compiler used for testing. The
motivation behind this is to allow upstreaming
https://github.com/swiftlang/llvm-project/pull/11835 but with the tests
disabled in upstream because the full implementation of -fbounds-safety
isn't available in Clang yet.

For shell tests when -fbounds-safety is available the
`clang-bounds-safety` feature is available which means tests can be
annotated with `# REQUIRES: clang-bounds-safety`.

API tests that need -fbounds-safety support in the compiler can use the
new `@skipUnlessBoundsSafety` decorator.

rdar://165225507

During the initialization sequence in our tests the first 'threads'
response sould only be kept if the process is actually stopped,
otherwise we will have stale data.

In VSCode, during the debug session startup sequence immediately after
'configurationDone' a 'threads' request is made. This initial request is
to retrieve the main threads name and id so the UI can be populated.
However, in our tests we do not want to cache this value unless the
process is actually stopped. We do need to make this initial request
because lldb-dap is caching the initial thread list during
configurationDone before the process is resumed. We need to make this
call to ensure the cached initial threads are purged.

I noticed this in a CI job for another review
(https://github.com/llvm/llvm-project/actions/runs/19348261989/job/55353961798)
where the tests incorrectly failed to fetch the threads prior to
validating the thread names.

https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/lldb-cmake-sanitized/2744/consoleText

Adding structured types for the evaluate request handler.

This should be mostly a non-functional change. I did catch some spelling
mistakes in our tests ('variable' vs 'variables').

Otherwise these tests are reliant on the compiler defaulting to having the extensions on.
Rest of LLVM's codebase doesn't seem to make such assumptions.

Tested by building with `-std=c2y` in Clang's C frotend's config file.

From
https://github.com/llvm/llvm-project/pull/148554#issuecomment-3083261858
- this adds an option for API tests to be run with the both PDB readers
on Windows. As there are a lot of failures with PDB, this is an opt-in
per test.

To get PDB, `-g -gcodeview` has to be used on Clang. `-gcodeview` alone
isn't enough, because it won't cause clang to pass `-debug` to the
linker.

#149498 tracks the (currently) failing tests.

In lldb-dap tests, we sometimes spawn subprocesses directly but do not
always correctly clean them up.

This can cause some tests, like the `TestDAP_disconnect.test_attach` to
hang and not properly respect timeouts.

To fix this, I am passing the `lldbtest.Base.spawnSubprocess` helper to
the adapter client so it can be used spawn subprocesses in a way that we
can ensure they're cleaned up.

Noticed this while looking into test stability that the 'entry' stop
reason is not triggering correctly. This should ensure we correctly
trigger the 'entry' stop reason when launching a process with
`"stopOnEntry": true`. I've also updated the tests to ensure we receive
the 'entry' stop reason to catch this regression.