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/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 858; // Replace inherited types with protocols in protocol layout
+const uint16_t SWIFTMODULE_VERSION_MINOR = 859; // channel check
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -860,6 +860,7 @@ namespace control_block {
     TARGET,
     SDK_NAME,
     REVISION,
+    CHANNEL,
     IS_OSSA,
     ALLOWABLE_CLIENT_NAME,
     HAS_NONCOPYABLE_GENERICS,
@@ -898,6 +899,11 @@ namespace control_block {
     BCBlob
   >;
 
+  using ChannelLayout = BCRecordLayout<
+    CHANNEL,
+    BCBlob
+  >;
+
   using IsOSSALayout = BCRecordLayout<
     IS_OSSA,
     BCFixed<1>