[Modules] Make r180934 more efficient by only loading top-level module maps in system header directories.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181643 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Lex/ModuleMap.cpp b/lib/Lex/ModuleMap.cpp
index de23485..62b1fc7 100644
--- a/lib/Lex/ModuleMap.cpp
+++ b/lib/Lex/ModuleMap.cpp
@@ -183,8 +183,7 @@
   // specific module (e.g., in /usr/include).
   if (File->getDir() == BuiltinIncludeDir &&
       isBuiltinHeader(llvm::sys::path::filename(File->getName()))) {
-    SmallVector<Module *, 4> AllModules;
-    HeaderInfo.collectAllModules(AllModules);
+    HeaderInfo.loadTopLevelSystemModules();
 
     // Check again.
     Known = Headers.find(File);