Deserialize Swift compatibility version in CompilerInvocation::loadFromSerializedAST()

LLDB needs the -swift-version because the -D__swift__ macro affects
how Clang modules are built. This currently has the really odd effect
that when debugging a Swift program that is not using the very latest
Swift version, the first "po" takes several seconds, because the
module cache needs to be rebuilt.

rdar://problem/40241256

lib/Frontend/CompilerInvocation.cpp

@@ -1203,6 +1203,7 @@ CompilerInvocation::loadFromSerializedAST(StringRef data) {
   if (info.status != serialization::Status::Valid)
     return info.status;
 
+  LangOpts.EffectiveLanguageVersion = info.compatibilityVersion;
   setTargetTriple(info.targetTriple);
   if (!extendedInfo.getSDKPath().empty())
     setSDKPath(extendedInfo.getSDKPath());