[Completion] Fix trailing closure index check

`Res.FirstTrailingClosureIndex` is an index into
the argument list, so comparing it against a
parameter index is wrong. Instead, compare it
against the argument index of the completion token,
which is what we want to be checking. This ensures
we don't try and offer argument completions for
non-function default arguments.

rdar://127760308

Author: hamishknight

lib/IDE/ArgumentCompletion.cpp

@@ -50,7 +50,8 @@ bool ArgumentTypeCheckCompletionCallback::addPossibleParams(
     const AnyFunctionType::Param *TypeParam = &ParamsToPass[Idx];
     bool Required = !Res.DeclParamIsOptional[Idx];
 
-    if (Res.FirstTrailingClosureIndex && Idx > *Res.FirstTrailingClosureIndex &&
+    if (Res.FirstTrailingClosureIndex &&
+        Res.ArgIdx > *Res.FirstTrailingClosureIndex &&
         !TypeParam->getPlainType()
              ->lookThroughAllOptionalTypes()
              ->is<AnyFunctionType>()) {

test/IDE/complete_multiple_trailingclosure.swift

@@ -18,6 +18,40 @@ func testGlobalFunc() {
 // GLOBALFUNC_AFTERLABEL-NOT: Begin completions
 }
 
+struct S {
+  func member() {}
+}
+func globalFunc2(x: Int = 0, _ fn: () -> Void) -> S {}
+func globalFunc3(x: Int, _ fn: () -> Void) -> S {}
+
+func testNonFuncArg() {
+  do {
+    // Don't complete for a non-function default argument.
+    globalFunc2 {} #^GLOBALFUNC2^#
+    // GLOBALFUNC2:     Begin completions, 2 items
+    // GLOBALFUNC2-DAG: Decl[InstanceMethod]/CurrNominal:   .member()[#Void#]; name=member()
+    // GLOBALFUNC2-DAG: Keyword[self]/CurrNominal: .self[#S#]; name=self
+
+    globalFunc2 {}
+      .#^GLOBALFUNC2_DOT^#
+    // GLOBALFUNC2_DOT:     Begin completions, 2 items
+    // GLOBALFUNC2_DOT-DAG: Decl[InstanceMethod]/CurrNominal:   member()[#Void#]; name=member()
+    // GLOBALFUNC2_DOT-DAG: Keyword[self]/CurrNominal: self[#S#]; name=self
+  }
+  do {
+    globalFunc3 {} #^GLOBALFUNC3^#
+    // GLOBALFUNC3:     Begin completions, 2 items
+    // GLOBALFUNC3-DAG: Decl[InstanceMethod]/CurrNominal:   .member()[#Void#]; name=member()
+    // GLOBALFUNC3-DAG: Keyword[self]/CurrNominal: .self[#S#]; name=self
+    
+    globalFunc3 {}
+      .#^GLOBALFUNC3_DOT^#
+    // GLOBALFUNC3_DOT:     Begin completions, 2 items
+    // GLOBALFUNC3_DOT-DAG: Decl[InstanceMethod]/CurrNominal:   member()[#Void#]; name=member()
+    // GLOBALFUNC3_DOT-DAG: Keyword[self]/CurrNominal: self[#S#]; name=self
+  }
+}
+
 struct SimpleEnum {
   case foo, bar
 
@@ -182,10 +216,9 @@ func testFallbackPostfix() {
   let _ = MyStruct4 {
     1
   } #^INIT_FALLBACK_1^#
-// INIT_FALLBACK_1: Begin completions, 3 items
+// INIT_FALLBACK_1: Begin completions, 2 items
 // INIT_FALLBACK_1-DAG: Keyword[self]/CurrNominal:          .self[#MyStruct4<Int>#]; name=self
 // INIT_FALLBACK_1-DAG: Decl[InstanceMethod]/CurrNominal/TypeRelation[Invalid]: .testStructMethod()[#Void#]; name=testStructMethod()
-// INIT_FALLBACK_1-DAG: Pattern/Local/Flair[ArgLabels]:     {#arg1: Int#}[#Int#]; name=arg1:
   let _ = MyStruct4(name: "test") {
     ""
   } arg3: {