swift234

Imbue ASTContext with a lazy resolver.

Have TypeChecker's constructor register itself as the lazy resolver,
and its destructor unregister itself. This introduces the
completely-sensible restriction that there can only be one type
checker active for an ASTContext at a time, which is the case already.

Use ASTContext's lazy resolver in a single place that's been causing
trouble (rdar://problem/20363958, rdar://problem/19773096), where
deserializing a protocol conformance can cause us to pass a null lazy
resolver into the protocol conformance table, which doesn't handle it
well. This commit fixes those issues, which I'm unable to reduce down
to a sane-enough test case to commit.

This commit implies a ton of cleanup work to eliminate LazyResolver
parameters from *everywhere*, deriving them from the ASTContext in the
few places they're needed as well. It's a good direction, but that
cleanup can be evolutionary.

Swift SVN r26824