ThinLTO: handles modules with empty summaries

We need to add an entry in the combined-index for modules that have
a hash but otherwise empty summary, this is needed so that we can
get the hash for the module.

Also, if no entry is present in the combined index for a module, we
need to skip it when trying to compute a cache entry.

Differential Revision: https://reviews.llvm.org/D25300

llvm-svn: 283654

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6147,8 +6147,8 @@ std::error_code ModuleSummaryIndexBitcodeReader::parseModule() {
           if (!TheIndex)
             break;
           if (TheIndex->modulePaths().empty())
-            // Does not have any summary emitted.
-            break;
+            // We always seed the index with the module.
+            TheIndex->addModulePath(Buffer->getBufferIdentifier(), 0);
           if (TheIndex->modulePaths().size() != 1)
             return error("Don't expect multiple modules defined?");
           auto &Hash = TheIndex->modulePaths().begin()->second.second;