[CSSPGO] Use preinliner decision by default when available

For CSSPGO, turn on `sample-profile-use-preinliner` by default. This simplifies the use of llvm-profgen preinliner as it's now simply driven by ContextShouldBeInlined flag for each context profile without needing extra compiler switch.

Note that llvm-profgen's preinliner is still off by default, under switch `csspgo-preinliner`.

Differential Revision: https://reviews.llvm.org/D109111