[libc++][CI] Replace LLDB test targets with libc++ test category (#110856)

We've been increasing the coverage of libc++ LLDB tests in the pre-merge
CI (see https://github.com/llvm/llvm-project/pull/110570). Unfortunately
the tests are spread across different targets. It would be great if we
had a single target that libc++ maintainers could run.

We do this by passing the `libc++` test-category as a parameter to
LLDB's [`dotest` testing
framework](https://lldb.llvm.org/resources/test.html). This will only
run LLDB tests that have been marked as belonging to the `libc++`
category.

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