[serialization] Diagnose loading modules from older Swifts.

...with a better message than the generic "older version of the
compiler" one, when we know it's actually a different version of
Swift proper.

This still uses the same internal module version numbers to check
if the module is compatible; the presentation of language versions
is a diagnostic thing only.

Speaking of module version numbers, this deliberately does NOT
increment VERSION_MINOR; it's implemented in a backwards-compatible
way.

This will only work going forwards, of course; all existing modules
don't have a short version string, and I don't feel comfortable
assuming all older modules we might encounter are "Swift 2.2".

rdar://problem/25680392

lib/Serialization/ModuleFile.cpp

@@ -190,6 +190,11 @@ validateControlBlock(llvm::BitstreamCursor &cursor,
         }
       }
 
+      // This field was added later; be resilient against its absence.
+      if (scratch.size() > 2) {
+        result.shortVersion = blobData.slice(0, scratch[2]);
+      }
+
       versionSeen = true;
       break;
     }
@@ -743,7 +748,7 @@ static bool isTargetTooNew(const llvm::Triple &moduleTarget,
 ModuleFile::ModuleFile(
     std::unique_ptr<llvm::MemoryBuffer> moduleInputBuffer,
     std::unique_ptr<llvm::MemoryBuffer> moduleDocInputBuffer,
-    bool isFramework,
+    bool isFramework, serialization::ValidationInfo &info,
     serialization::ExtendedValidationInfo *extInfo)
     : ModuleInputBuffer(std::move(moduleInputBuffer)),
       ModuleDocInputBuffer(std::move(moduleDocInputBuffer)),
@@ -775,7 +780,7 @@ ModuleFile::ModuleFile(
     case CONTROL_BLOCK_ID: {
       cursor.EnterSubBlock(CONTROL_BLOCK_ID);
 
-      auto info = validateControlBlock(cursor, scratch, extInfo);
+      info = validateControlBlock(cursor, scratch, extInfo);
       if (info.status != Status::Valid) {
         error(info.status);
         return;