[LTO] Use .at instead of .lookup to avoid copies.

`DenseMap::lookup` returns by value (because it default-creates the
returned value if the key isn't present in the map), which means that we
do a lot of copying here. Since we assert that something is present in
the returned value two lines below this call, it's safe to use `.at`
here instead.

Copying and destroying the dense maps here is responsible for 60% of the
time spent in LTO indexing in a large internal build.

Author: amharc

llvm/lib/Transforms/IPO/FunctionImport.cpp

@@ -1540,8 +1540,7 @@ void llvm::gatherImportedSummariesForModule(
     auto &SummariesForIndex =
         LookupOrCreate(ModuleToSummariesForIndex, FromModule);
 
-    const auto &DefinedGVSummaries =
-        ModuleToDefinedGVSummaries.lookup(FromModule);
+    const auto &DefinedGVSummaries = ModuleToDefinedGVSummaries.at(FromModule);
     const auto &DS = DefinedGVSummaries.find(GUID);
     assert(DS != DefinedGVSummaries.end() &&
            "Expected a defined summary for imported global value");