[libclang] Do index 'extern' declarations inside functions.

rdar://12257073

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@163563 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/libclang/IndexDecl.cpp b/tools/libclang/IndexDecl.cpp
index 4c78f5e..53a98f1 100644
--- a/tools/libclang/IndexDecl.cpp
+++ b/tools/libclang/IndexDecl.cpp
@@ -325,7 +325,7 @@
   }
 }
 
-void IndexingContext::indexTopLevelDecl(Decl *D) {
+void IndexingContext::indexTopLevelDecl(const Decl *D) {
   if (isNotFromSourceFile(D->getLocation()))
     return;