Revert "[Driver] Default CLANG_DEFAULT_PIE_ON_LINUX to ON"

See post-commit discussion on https://reviews.llvm.org/D120305.
This change breaks the clang-ppc64le-rhel buildbot, though
there is suspicion that it's an issue with the bot. The change
also had a larger than expected impact on compile-time and
code-size.

This reverts commit 3c4ed026
and some followup changes.