[Parse] Tighten constraints in canParseNonisolatedAsTypeModifier

Accept it only if it's spelled `nonisolated(nonsending)` and nothing
else to minimize any possible source compatibility impact.

Author: xedin

lib/Parse/ParseType.cpp

@@ -1749,17 +1749,19 @@ bool Parser::canParseNonisolatedAsTypeModifier() {
   if (Tok.isAtStartOfLine())
     return false;
 
-  // Always requires `(<<argument>>)`
-  if (!Tok.is(tok::l_paren))
+  // Always requires `(nonsending)`, together
+  // we don't want eagerly interpret something
+  // like `nonisolated(0)` as a modifier.
+
+  if (!consumeIf(tok::l_paren))
     return false;
 
-  // Consume argument list
-  skipSingle();
+  if (!Tok.isContextualKeyword("nonsending"))
+    return false;
 
-  // if consumed '(...)' ended up being followed
-  // by `[async, throws, ...] -> ...` this means
-  // the `nonisolated` is invalid as a modifier.
-  return !isAtFunctionTypeArrow();
+  consumeToken();
+
+  return consumeIf(tok::r_paren);
 }
 
 bool Parser::canParseTypeScalar() {

test/Parse/execution_behavior_attrs.swift

@@ -77,3 +77,10 @@ do {
 
   _ = [nonisolated()]
 }
+
+do {
+  func nonisolated(_: Int) -> Int { 42 }
+
+  nonisolated(0) // expected-warning {{result of call to 'nonisolated' is unused}}
+  print("hello")
+}