[@semantics] add SemanticsAttr to SILFunction.

Enable SIL parsing and SIL serialization of semantics.

We add one more field to SILFunctionLayout for semantics. We should refactor
handling of attributes at SIL level, right now they are in SILFunction as bool
or std::string and in SIL serializer as a 1-bit field or an ID field.

rdar://17525564


Swift SVN r19434

lib/SIL/SILPrinter.cpp

@@ -1308,6 +1308,9 @@ void SILFunction::print(llvm::raw_ostream &OS, bool Verbose) const {
   if (isNoinline())
     OS << "[noinline] ";
 
+  if (!getSemanticsAttr().empty())
+    OS << "[semantics \"" << getSemanticsAttr() << "\"] ";
+
   printName(OS);
   OS << " : $";