[Driver] Default CLANG_DEFAULT_PIE_ON_LINUX to ON

Default the option introduced in D113372 to ON to match all(?) major Linux
distros. This matches GCC and improves consistency with Android and linux-musl
which always default to PIE.
Note: CLANG_DEFAULT_PIE_ON_LINUX will be removed in the future.

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D120305