[Parse] Register a syntax context while parsing availability macros

Author: lorentey

lib/Parse/ParseDecl.cpp

@@ -1632,6 +1632,9 @@ Parser::parseAvailabilityMacro(SmallVectorImpl<AvailabilitySpec *> &Specs) {
   if (NameMatch == Map.end())
     return makeParserSuccess(); // No match, it could be a standard platform.
 
+  SyntaxParsingContext VersionRestrictionContext(
+      SyntaxContext, SyntaxKind::AvailabilityVersionRestriction);
+
   consumeToken();
 
   llvm::VersionTuple Version;