[Index] Write stdlib records in alphabetical order

The order for writing records of the stdlib currently depends on
`StringMap` iteration (in a slightly roundabout manner). Sort these
alphabetically instead.

Author: bnbarham

lib/Index/IndexRecord.cpp

@@ -233,9 +233,15 @@ class StdlibGroupsIndexRecordingConsumer : public IndexDataConsumer {
   }
 
   void finish() override {
-    for (auto &pair : TrackerByGroup) {
-      StringRef groupName = pair.first();
-      SymbolTracker &tracker = *pair.second;
+    SmallVector<std::pair<StringRef, SymbolTracker &>, 0> SortedGroups;
+    for (auto &entry : TrackerByGroup) {
+      SortedGroups.emplace_back(entry.first(), *entry.second);
+    }
+    llvm::sort(SortedGroups, llvm::less_first());
+
+    for (auto &pair : SortedGroups) {
+      StringRef groupName = pair.first;
+      SymbolTracker &tracker = pair.second;
       bool cont = onFinish(groupName, tracker);
       if (!cont)
         break;

test/Index/Store/unit-one-file-multi-file-invocation.swift

@@ -18,8 +18,8 @@
 
 // CHECK: [[SWIFT]]
 // CHECK: DEPEND START
-// CHECK: Record | system | Swift.String | [[MODULE]] | {{.+}}.swiftinterface_String-{{.*}}
 // CHECK: Record | system | Swift.Math.Floating | [[MODULE]] | {{.+}}.swiftinterface_Math_Floating-{{.*}}
+// CHECK: Record | system | Swift.String | [[MODULE]] | {{.+}}.swiftinterface_String-{{.*}}
 // CHECK: DEPEND END
 
 func test1() {