swift234

AST/SILGen: Factor out "function body might be inlined in other resilience domains" logic

If a function is public, and either @_transparent or @inline(__always),
we need to make its body available for inlining in other resilience
domains. The more general concept here is an 'inlineable' function;
once the precise behaviors we want are nailed down, the set of AST
attributes for exposing this will likely change.

At the SIL level, inlineable functions are marked with the [fragile]
attribute. The SIL serializer only serializes [fragile] functions
unless -sil-serialize-all is passed in.

This patch fixes two problems in this area by consolidating some
duplicated logic:

1) Property accesses in Sema did not check for @inline(__always)
   functions, or functions nested inside inlineable functions.
   This manifested as IRGen crashes if an inlineable function
   accessed a property of a resilient type.

2) In SILGen, functions nested inside [fragile] functions were
   properly [fragile], but @inline(__always) was not taken into
   account. This manifested as SIL serializer crashes where a
   [fragile] function could reference a non-public, non-[fragile]
   function.

This change is part of the series for building the standard library
without -sil-serialize-all.