[lldb-dev] check-lldb will start using in-tree clang by default

[Pavel Labath via lldb-dev]

I am going to check in a change (D39215) which causes the check-lldb
target to use the just-built clang for compiling the test inferiors
(instead of the system compiler, which was the old default). The main
reason for this is to provide better reproducibility of test results
between different machines/developers, by removing one of the main
sources of nondeterminism. This behavior can be overridden by setting
the LLDB_TEST_C_COMPILER and LLDB_TEST_CXX_COMPILER cmake variables.

For the change to take effect you will need to clean your build folder
(or at least, remove the affected variables from your CMakeCache.txt).
After this you may observe a change in the test results from the
check-lldb run.

Note that this change only affect cmake code -- if you run your tests
by running dotest.py directly, nothing will change for you.

regards,
pavel