swift234

Teach the ColdBlocks SIL analysis to recognize the branch hint semantic model.

We're temporarily using @semantics until we have mandatory inlining of branch hints.

We now have @noinline, which helps a lot, but:
- @noinline and "cold/slow" are not the same thing.

- Some functions may need to be inlined into hot paths, but that
  doesn't mean we should also inline them into cold paths.

- It is easier to find cold blocks than to look for blocks that
  contain calls to @noinline functions. And that doesn't necessarilly
  mean the blocks are cold anyway.

Swift SVN r19455