swift234

Move bridging conversions into the type checker.

Previously, bridged value types and their corresponding Objective-C
classes allow inter-conversion via a number of user-defined conversion
functions in the Foundation module. Instead, make this a general
feature of the type checker so we can reason about it more
directly. Fixes <rdar://problem/16956098> and
<rdar://problem/17134986>, and eliminates 11 (half) of the
__conversion functions from the standard library and overlays.

A few notes:
  - The XCTest changes are because a String can no longer directly
  conform to CVarArg: this is a Good Thing (TM), because it should be
  ambiguous: did you mean to pass it as an NSString or a C string?
  - The Objective-C representations for the bridged collections are
  hard-coded in the type checker. This is unfortunate and can be
  remedied by adding another associated type to the
  _BridgedToObjectiveC protocol.

Swift SVN r19618