Use StringRef in more places to match recent llvm changes.

llvm r283043 and possibly other recent changes switch to use StringRef
instead of char* pointers. Update Swift to match. In some cases, this is
a clear improvement. It would be good to assess the impact on memory use,
particularly for the Filename component of source locations.

Note that the change to SILLocation::isNull fixes an apparent bug where
the location was treated as null when the filename was *not* null.

lib/Serialization/Serialization.cpp

@@ -3598,8 +3598,8 @@ class DeclGroupNameContext {
       auto PathOp = VD->getDeclContext()->getParentSourceFile()->getBufferID();
       if (!PathOp.hasValue())
         return NullGroupName;
-      StringRef FullPath = StringRef(VD->getASTContext().SourceMgr.
-                                     getIdentifierForBuffer(PathOp.getValue()));
+      StringRef FullPath =
+        VD->getASTContext().SourceMgr.getIdentifierForBuffer(PathOp.getValue());
       if (!pMap) {
         YamlGroupInputParser Parser(RecordPath);
         if (!Parser.parse()) {