[SourceKit] Handle macro arguments in placeholder expansion

Allow macro argument placeholders to expand into trailing closures.

rdar://150550747

Author: hamishknight

test/SourceKit/CodeExpand/code-expand.swift

@@ -273,3 +273,22 @@ expandClosureWithInternalParameterNames {
 // CHECK: withtrail { a, b in
 // CHECK-NEXT: <#code#>
 }
+
+func expandMacro() {
+  #foo(<#T##() -> Int#>)
+  // CHECK:      #foo {
+  // CHECK-NEXT:   <#code#>
+  // CHECK-NEXT: }
+
+  #foo(bar: <#T##() -> ()#>)
+  // CHECK:      #foo {
+  // CHECK-NEXT:   <#code#>
+  // CHECK-NEXT: }
+
+  #foo(bar: <#T##() -> Int#>, baz: <#T##() -> ()#>)
+  // CHECK:      #foo {
+  // CHECK-NEXT:   <#code#>
+  // CHECK-NEXT: } baz: {
+  // CHECK-NEXT:   <#code#>
+  // CHECK-NEXT: }
+}

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1690,6 +1690,9 @@ class PlaceholderExpansionScanner {
         if (auto *CE = dyn_cast<CallExpr>(E)) {
           // Call expression can have argument.
           Args = CE->getArgs();
+        } else if (auto *ME = dyn_cast<MacroExpansionExpr>(E)) {
+          // Macros can have arguments.
+          Args = ME->getArgs();
         }
         if (!Args)
           return false;