Piggybacks some resilience diagnostics onto the availability
checking code.
Public and versioned functions with inlineable bodies can only
reference other public and internal entities, since the SIL code
for the function body is serialized and stored as part of the
module.
This includes @_transparent functions, @_inlineable functions,
accessors for @_inlineable storage, @inline(__always) functions,
and in Swift 4 mode, default argument expressions.
The new checks are a source-breaking change, however we don't
guarantee source compatibility for underscored attributes.
The new ABI and tests for the default argument model will come in
subsequent commits.
This will let the performance inliner inline a function even if the costs are too high.
This attribute is only a hint to the inliner.
If the inliner has other good reasons not to inline a function,
it will ignore this attribute. For example if it is a recursive function (which is
currently not supported by the inliner).
Note that setting the inline threshold to 0 does disable performance inlining at all and in
this case also the @inline(__always) has no effect.
Swift SVN r21452
