Simplify FindExternalVisibleDeclsByName by making it return a bool indicating
if it found any decls, rather than returning a list of found decls. This
removes a returning-ArrayRef-to-deleted-storage bug from
MultiplexExternalSemaSource (in code not exercised by any of the clang
binaries), reduces the work required in the found-no-decls case with PCH, and
importantly removes the need for DeclContext::lookup to be reentrant.

No functionality change intended!

llvm-svn: 174576
diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
index 6b9fe26..96ebe92 100644
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -43,10 +43,10 @@
   return 0;
 }
 
-DeclContextLookupResult 
+bool
 ExternalASTSource::FindExternalVisibleDeclsByName(const DeclContext *DC,
                                                   DeclarationName Name) {
-  return DeclContext::lookup_result();
+  return false;
 }
 
 void ExternalASTSource::completeVisibleDeclsMap(const DeclContext *DC) {