This argument allows to set specific sysroot pass which will be used for
building LLDB API test programs.
It might come in handy for setting up cross-platform remote runs of API
tests on Windows host.
It can be useful for cross-compiling LLDB API tests. The argument can be
set using `LLDB_TEST_USER_ARGS` argument:
```
cmake ...
-DLLDB_TEST_USER_ARGS="...;--sysroot;C:\path\to\sysroot;..."
...
```
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