[AutoDiff] Allow more VJP inlining decisions (#88922)
Prior to regular inlining decision logic, VJP callees have additional
checks since some inlining might harm further AutoDiff Closure
Specialization (ADCS) pass logic. This patch relaxes some of these
restrictions, allowing more VJP inlining decisions to be taken while
still not harming the ADCS optimizations.
VJP inlining decisions newly allowed:
1) potential inlining during HighLevel pass pipeline (previously - hard
no);
2) derivative thunks callees to any callers;
3) trivial VJPs (no other VJPs called, only top-level pullback) to any
callers;
4) any VJPs to VJP callers with control flow which only use the pullback
from current apply site directly in their top-level pullback.
This patch also introduces best-effort attempt of detection of explicit
derivatives by looking at the corresponding `AbstractFunctionDecl` and
looking for a `derivative` attr with reverse-mode.
