[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