This PR adds support for running the LLDB test suite under MTE. It's
enabled by default on capable hardware when asserts are enabled. It
relies on a launcher (#185921) which launches the process with the
appropriate posix_spawn attribute. One thing worth noting here is that
child processes inherit the MTE property, so binaries launched by the
test suite in this mode also run under MTE.
Besides the logic to detect the default and thread through the launcher,
I also had to make a small change to LLVM LIT's `ToolSubst` class to
support an optional launcher for the shell tests.
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