[sil] Add the ability to mark a function with isolation just so we can write more concurrency tests in SIL.

Specifically, we write a string out like:

sil [isolation "$REPRESENTATION OF ISOLATION"] @function : $@convention(thin) ...

The idea is that by using a string, we avoid parsing issues of the isolation and
have flexibility. I left in the way we put isolation into the comment above
functions so I did not break any tests that rely on it. I also made it so that
we only accept this with sil tests that pass in the flag
"sil-print-function-isolation-info". I am going to for the next release put in a
full real implementation of this that allows for actor isolation to become a
true first class citizen in SIL. But for now this at least lets us write tests
in the short term.

Since this is temporary and behind a flag, I did not add support for
serialization since this is just for writing textual SIL tests.

lib/SIL/IR/SILPrinter.cpp

@@ -95,6 +95,11 @@ llvm::cl::opt<bool> SILPrintGenericSpecializationInfo(
     llvm::cl::desc("Include generic specialization"
                    "information info in SIL output"));
 
+llvm::cl::opt<bool> SILPrintFunctionIsolationInfo(
+    "sil-print-function-isolation-info", llvm::cl::init(false),
+    llvm::cl::desc("Print out isolation info on functions in a manner that SIL "
+                   "understands [e.x.: not in comments]"));
+
 static std::string demangleSymbol(StringRef Name) {
   if (SILFullDemangle)
     return Demangle::demangleSymbolAsString(Name);
@@ -3630,6 +3635,15 @@ void SILFunction::print(SILPrintContext &PrintCtx) const {
     OS << "[available " << availability.getVersionString() << "] ";
   }
 
+  // This is here only for testing purposes.
+  if (SILPrintFunctionIsolationInfo) {
+    if (auto isolation = getActorIsolation()) {
+      OS << "[isolation \"";
+      isolation->printForSIL(OS);
+      OS << "\"] ";
+    }
+  }
+
   switch (getInlineStrategy()) {
     case NoInline: OS << "[noinline] "; break;
     case AlwaysInline: OS << "[always_inline] "; break;