swift234

SILGen: Only emit implicit and Clang importer decls by need.

If a SILDeclRef references a decl that isn't explicit in the source code and can't be referenced externally, then we only need to emit it if it's referenced in the current TU. Partially addresses rdar://problem/21444126, though we still eagerly generate witness tables for generated conformances, which still pull in a bunch of noise.

Swift SVN r29536