Parse #isolation as CurrentContextIsolationExpr when we don't have swift-syntax

When building without swift-syntax, we don't have the ability to handle
macros. However, the concurrency runtime makes use of `#isolation` in
a number of places. To continue supporting building without
swift-syntax, parse `#isolation` directly into the otherwise-unspellable
CurrentContextIsolationExpr but *only* when swift-syntax is not available.

Author: DougGregor

lib/Parse/ParseExpr.cpp

@@ -3454,6 +3454,20 @@ ParserResult<Expr> Parser::parseExprMacroExpansion(bool isExprBasic) {
   if (!macroNameRef)
     return status;
 
+#if !SWIFT_BUILD_SWIFT_SYNTAX
+  // If we don't have swift-syntax and therefore have no support for macros,
+  // recognize #isolation as special and route it through
+  // CurrentContextIsolationExpr.
+  if (macroNameRef.getBaseName().userFacingName() == "isolation" &&
+      genericArgs.empty() &&
+      (!argList || argList->empty())) {
+    return makeParserResult(
+      status,
+      new (Context) CurrentContextIsolationExpr(
+          macroNameLoc.getStartLoc(), Type()));
+  }
+#endif
+
   return makeParserResult(
       status,
       MacroExpansionExpr::create(