swift234

SILGen: Re-work C function pointer conversion diagnostics

This removes the partially-correct ABI check in Sema and diagnoses
unsupported conversions in SILGen instead. The new check is more
accurate and correctly diagnoses conversions of DeclRef's to
ABI-incompatible @convention(c) types.

This also fixes two cases where we used to crash but could instead
emit a trivial cast:

- Conversions between ABI-compatible (but not identical)
  @convention(c) types
- Conversions of a DeclRef to an ABI-compatible (but not identical)
  @convention(c) type

Fixes <rdar://problem/22470105>.

Swift SVN r32163