Mangle imported declarations using their C names.

This makes them consistent no matter what shenanigans are pulled by
the importer, particularly NS_ENUM vs. NS_OPTIONS and NS_SWIFT_NAME.

The 'NSErrorDomain' API note /nearly/ works with this, but the
synthesized error struct is still mangled as a Swift declaration,
which means it's not rename-stable. See follow-up commits.

The main place where this still falls down is NS_STRING_ENUM: when
this is applied, a typedef is imported as a unique struct, but without
it it's just a typealias for the underlying type. There's also still a
problem with synthesized conformances, which have a module mangled
into the witness table symbol even though that symbol is linkonce_odr.

rdar://problem/31616162

test/stdlib/Reflection_objc.swift

@@ -63,7 +63,7 @@ print("ObjC quick look objects:")
 // CHECK-LABEL: ObjC enums:
 print("ObjC enums:")
 
-// CHECK-NEXT: We cannot reflect ComparisonResult yet
+// CHECK-NEXT: We cannot reflect NSComparisonResult yet
 print("We cannot reflect \(ComparisonResult.orderedAscending) yet")
 
 // Don't crash when introspecting framework types such as NSURL.