Serialization: restrict swiftmodules to distribution channels

There are scenarios where different compilers are distributed with
compatible serialization format versions and the same tag. Distinguish
swiftmodules in such a case by assigning them to different distribution
channels. A compiler expecting a specific channel will only read
swiftmodules from the same channel. The channels should be defined by
downstream code as it is by definition vendor specific.

For development, a no-channel compiler loads or defining the env var
SWIFT_IGNORE_SWIFTMODULE_REVISION skips this new check.

rdar://123731777

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -316,6 +316,8 @@ struct ModuleRebuildInfo {
       return "compiled with a newer version of the compiler";
     case Status::RevisionIncompatible:
       return "compiled with a different version of the compiler";
+    case Status::ChannelIncompatible:
+      return "compiled for a different distribution channel";
     case Status::NotInOSSA:
       return "module was not built with OSSA";
     case Status::NoncopyableGenericsMismatch: