Change how replacements are filtered (#541)

This replaces the use of `isExpansionInMainFile` with instead taking the list of input files, and only touching them. This doesn't keep `isExpansionInMainFile` because it should be redundant, and as such it'd be easy to forget. Unless it starts being a performance issue, it's probably better to omit.

migrate_cpp/cpp_refactoring/fn_inserter.cpp

@@ -13,13 +13,8 @@ namespace Carbon {
 FnInserter::FnInserter(std::map<std::string, Replacements>& in_replacements,
                        cam::MatchFinder* finder)
     : Matcher(in_replacements) {
-  // TODO: Switch from isExpansionInMainFile to isDefinition. That should then
-  // include `for (const auto* redecl : func->redecls())` to generate
-  // replacements.
-  finder->addMatcher(
-      cam::functionDecl(cam::isExpansionInMainFile(), cam::hasTrailingReturn())
-          .bind(Label),
-      this);
+  finder->addMatcher(cam::functionDecl(cam::hasTrailingReturn()).bind(Label),
+                     this);
 }
 
 void FnInserter::run(const cam::MatchFinder::MatchResult& result) {