Fixup Use-of-Temporary

The Optional parameter here was being copied instead of being taken by
const reference. The expectation is that we call this function and
extract the data from a ModuleDepGraphNode node, but instead we were
extracting a copy which would promptly blow up.

Thanks ASAN!

Author: CodaFi

include/swift/Driver/FineGrainedDependencyDriverGraph.h

@@ -466,7 +466,7 @@ class ModuleDepGraph {
   void printPath(raw_ostream &out, const driver::Job *node) const;
 
   /// Get a printable filename, given a node's swiftDeps.
-  StringRef getProvidingFilename(Optional<std::string> swiftDeps) const;
+  StringRef getProvidingFilename(const Optional<std::string> &swiftDeps) const;
 
   /// Print one node on the dependency path.
   static void printOneNodeOfPath(raw_ostream &out, const DependencyKey &key,

lib/Driver/FineGrainedDependencyDriverGraph.cpp

@@ -749,7 +749,7 @@ void ModuleDepGraph::printPath(raw_ostream &out,
 }
 
 StringRef ModuleDepGraph::getProvidingFilename(
-    const Optional<std::string> swiftDeps) const {
+    const Optional<std::string> &swiftDeps) const {
   if (!swiftDeps)
     return "<unknown";
   auto ext = llvm::sys::path::extension(*swiftDeps);