[Macros] Handle macro argument lists in diagnostics

This improves diagnostics for missing/extra/incorrectly-labeled arguments.

Author: DougGregor

lib/Sema/CSDiagnostics.cpp

@@ -4753,7 +4753,8 @@ bool MissingArgumentsFailure::diagnoseSingleMissingArgument() const {
   auto anchor = getRawAnchor();
   if (!(isExpr<CallExpr>(anchor) || isExpr<SubscriptExpr>(anchor) ||
         isExpr<UnresolvedMemberExpr>(anchor) ||
-        isExpr<ObjectLiteralExpr>(anchor)))
+        isExpr<ObjectLiteralExpr>(anchor) ||
+        isExpr<MacroExpansionExpr>(anchor)))
     return false;
 
   if (SynthesizedArgs.size() != 1)
@@ -5112,6 +5113,8 @@ MissingArgumentsFailure::getCallInfo(ASTNode anchor) const {
     return std::make_pair((Expr *)SE, SE->getArgs());
   } else if (auto *OLE = getAsExpr<ObjectLiteralExpr>(anchor)) {
     return std::make_pair((Expr *)OLE, OLE->getArgs());
+  } else if (auto *ME = getAsExpr<MacroExpansionExpr>(anchor)) {
+    return std::make_pair((Expr *)ME, ME->getArgs());
   }
   return None;
 }

test/Macros/macros_diagnostics.swift

@@ -6,6 +6,7 @@
 // REQUIRES: executable_test
 
 @expression macro stringify<T>(_ value: T) -> (T, String) = MacroDefinition.StringifyMacro
+// expected-note@-1{{'stringify' declared here}}
 @expression macro missingMacro1(_: Any) = MissingModule.MissingType // expected-note{{'missingMacro1' declared here}}
 @expression macro missingMacro2(_: Any) = MissingModule.MissingType
 
@@ -40,6 +41,11 @@ struct ZZZ {
 func test(a: Int, b: Int) {
   // FIXME: Bad diagnostic.
   let s = #stringify<Int, Int>(a + b) // expected-error{{type of expression is ambiguous without more context}}
+
+  _ = #stringify()
+  // expected-error@-1{{missing argument for parameter #1 in call}}
+  _ = #stringify(label: a + b)
+  // expected-error@-1{{extraneous argument label 'label:' in call}}
 }
 
 func shadow(a: Int, b: Int, stringify: Int) {