[SIL] Add flag to SILFunction to indicate async.

Includes boilerplate to parse and print as well as to serialize and
deserialize.

include/swift/SIL/SILFunction.h

@@ -274,6 +274,9 @@ private:
   /// that it may have unboxed capture (i.e. @inout_aliasable parameters).
   unsigned IsWithoutActuallyEscapingThunk : 1;
 
+  /// True if this function is an async function.
+  unsigned IsAsync : 1;
+
   /// If != OptimizationMode::NotSet, the optimization mode specified with an
   /// function attribute.
   unsigned OptMode : NumOptimizationModeBits;
@@ -501,6 +504,10 @@ public:
     IsWithoutActuallyEscapingThunk = val;
   }
 
+  bool isAsync() const { return IsAsync; }
+
+  void setAsync(bool val = true) { IsAsync = val; }
+
   /// Returns the calling convention used by this entry point.
   SILFunctionTypeRepresentation getRepresentation() const {
     return getLoweredFunctionType()->getRepresentation();