[Sema/SILGen] Track per-source-file "used" conformances to reference in SILGen

Sema was dutifully tracking conformances that were "used" as part of
type checking, so it could make sure that those conformances got
completed for SILGen to use. However, this information never actually
made it to SILGen, which included its own (more conservative, not
broad enough) heuristics for finding "used" conformances. Teach Sema
to record conformances within the appropriate source file, and have
SILGen reference the conformances when it emits SIL for the source
file.

Author: DougGregor

test/IDE/newtype.swift

@@ -25,10 +25,10 @@
 // PRINT-NEXT:  extension Foo {
 // PRINT-NEXT:    static let err: ErrorDomain
 // PRINT-NEXT:  }
-// PRINT-NEXT:  struct ClosedEnum : RawRepresentable, _SwiftNewtypeWrapper {
-// PRINT-NEXT:    init(rawValue: String?)
-// PRINT-NEXT:    var _rawValue: NSString?
-// PRINT-NEXT:    var rawValue: String? { get }
+// PRINT-NEXT:  struct ClosedEnum : RawRepresentable, _SwiftNewtypeWrapper, Equatable, Hashable, Comparable, _ObjectiveCBridgeable {
+// PRINT-NEXT:    init(rawValue: String)
+// PRINT-NEXT:    var _rawValue: NSString
+// PRINT-NEXT:    var rawValue: String { get }
 // PRINT-NEXT:  }
 // PRINT-NEXT:  extension ClosedEnum {
 // PRINT-NEXT:    static let firstClosedEntryEnum: ClosedEnum