[lldb][test] Don't run libc++ API tests without a locally built libc++ (llvm#162657)

API tests in the `libc++` category will try their best to build against
a locally built libc++. If none exists, the `Makefile.rules` currently
fall back to using the system libc++.

The issue with falling back to the system libc++ is that we are now
potentially not testing what we intended to. But we also can't rely on
certain libc++ features being available that the tests are trying to
use. On Apple platforms this is a configuration error (because libc++ is
the only stdlib supported), but we can't make it an error on Linux
because a user might want to run the API tests with libstdc++.

The Ubunutu 22.04 bots on the Apple fork are failing to run following
tests are failing:
* `TestLibcxxInternalsRecognizer.py`
* `TestDataFormatterStdRangesRefView.py` because the system stdlib
doesn't have `std::ranges` support yet. And the tests just fail to
build. Building libc++ on those bots is also not possible because the
system compiler is too old (and the Apple fork builds all the
subprojects standalone, so it requires the system compiler).

This patch marks tests in the `libc++` category as `UNSUPPORTED` if no
local libc++ is available.

The downside is that we will inevitably lose coverage on bots that were
running these tests without a local libc++. Arguably those weren't
really testing the right thing. But for vendors with LLDB forks it might
have been useful to at least know that the tests on the fork don't fail
against the system libc++.

Confirmed that the libc++ pre-merge CI still runs these tests (since it
uses the explicit `--category libc++` dotest flag). Also confirmed that
LLDB pre-merge CI runs the tests (because it builds `libcxx` locally).

**Workaround**

If you do need want to run libc++ tests against the system stdlib, you
can invoke `lldb-dotest` with the `--category libc++` flag:
```
./path/to/build/lldb-dotest --category libc++

OR

./path/to/build/bin/llvm-lit -sv --param dotest-args='--category libc++' "/path/to/monorepo/lldb/test/API
```

rdar://136231390
(cherry picked from commit 4188e18)

Author: Michael137

lldb/packages/Python/lldbsuite/test/dotest.py

@@ -786,6 +786,10 @@ def canRunLibcxxTests():
         return True, "libc++ always present"
 
     if platform == "linux":
+        if not configuration.libcxx_include_dir or not configuration.libcxx_library_dir:
+            return False, "API tests require a locally built libc++."
+
+        # Make sure -stdlib=libc++ works since that's how the tests will be built.
         with tempfile.NamedTemporaryFile() as f:
             cmd = [configuration.compiler, "-xc++", "-stdlib=libc++", "-o", f.name, "-"]
             p = subprocess.Popen(