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/Basic/Version.cpp

@@ -313,6 +313,16 @@ StringRef getCurrentCompilerSerializationTag() {
 #endif
 }
 
+StringRef getCurrentCompilerChannel() {
+  static const char* forceDebugChannel =
+    ::getenv("SWIFT_FORCE_SWIFTMODULE_CHANNEL");
+  if (forceDebugChannel)
+    return forceDebugChannel;
+
+  // Leave it to downstream compilers to define the different channels.
+  return StringRef();
+}
+
 unsigned getUpcomingCxxInteropCompatVersion() {
   return SWIFT_VERSION_MAJOR + 1;
 }