Revert "[Driver] Enable to use C++20 standalne by -fcxx-modules"

This reverts commit a544710c.

See discussion in D120540.

This breaks C++ Clang modules on Darwin and also more than a dozen
tests in the LLDB testsuite.  I think we need to be more careful to
separate out the enabling of Clang C++ modules and C++20
modules. Either by having -fmodules-ts control the HaveModules flag,
or by adding a way to explicitly turn them off.