Thanks, @Manna, these changes look good to me!
I agree that this looks like a good change. CXXBaseSpecifier contains a SourceRange, a SourceLocation, a unsigned (used as a bit-field, and a TypeSourceInfo*. Those are all individually cheap to copy, but together are large enough to justify use of a reference.
This looks good too. The returned map value has const VarDecl* as a key and the element type is FixItList (aka SmallVector<FixItHint, 4>) . FixItHint contains two CharSourceRange objects, a std::string, and a bool, so we definitely don't want to be copying that; especially since the value isn't even wanted here!
Another case of CXXBaseSpecifier which, per earlier comments, is large enough to justify use of a reference.