swift234

SILGen: Clean up ErasureExpr lowering, NFC

This is a step towards partially-applying methods that return Self
on existentials.

- We model opening of both existential values and metatypes with
  OpenExistentialExpr, but erasure had two forms, ErasureExpr and
  MetatypeErasureExpr. Combine them into one, since both Sema and
  SILGen have similar code paths for each.

- If the source type of an ErasureExpr is a closed existential,
  have Sema emit an OpenExistentialExpr, and remove SILGen's
  openExistentialForErasure() path, which mostly duplicates
  openExistentialImpl().

- There was one difference between openExistentialForErasure() and
  openExistentialImpl(). The former would emit the opaque value in
  +0 context, and the latter in a +1 with initialization. The
  previous patch ensures that visitOpaqueValueExpr() generates
  equivalent code in both cases.

Swift SVN r31261