Xcode: modify lldb-python-test-suite target to build inferiors with $(LLDB_PYTHON_TESTSUITE_CC)

$(LLDB_PYTHON_TESTSUITE_CC) defaults to the just-built clang.  Together
with changes to the zorg repo, this enables the Green Dragon LLDB OS X
Xcode-based builder to run the new TSAN LLDB tests.

llvm-svn: 265315