[index] Fix forward declarations interfering with USR generation of external source symbols

Patch by Nathan Hawes.
https://reviews.llvm.org/D33346

llvm-svn: 303484
diff --git a/clang/lib/Index/IndexSymbol.cpp b/clang/lib/Index/IndexSymbol.cpp
index 0bfa193..a108f86 100644
--- a/clang/lib/Index/IndexSymbol.cpp
+++ b/clang/lib/Index/IndexSymbol.cpp
@@ -318,16 +318,7 @@
   if (Info.Properties & (unsigned)SymbolProperty::Generic)
     Info.Lang = SymbolLanguage::CXX;
 
-  auto getExternalSymAttr = [](const Decl *D) -> ExternalSourceSymbolAttr* {
-    if (auto *attr = D->getAttr<ExternalSourceSymbolAttr>())
-      return attr;
-    if (auto *dcd = dyn_cast<Decl>(D->getDeclContext())) {
-      if (auto *attr = dcd->getAttr<ExternalSourceSymbolAttr>())
-        return attr;
-    }
-    return nullptr;
-  };
-  if (auto *attr = getExternalSymAttr(D)) {
+  if (auto *attr = D->getExternalSourceSymbolAttr()) {
     if (attr->getLanguage() == "Swift")
       Info.Lang = SymbolLanguage::Swift;
   }