[Serialization] Filter Decl to deserialize by their attributes

Add an alternative to getTopLevelDecls and getDeclChecked to limit which
decls are deserialized by first looking at their attributes. If the
attributes are accepted by a function passed as argument the decl is
fully deserialized, otherwise it is ignored.

The filter is included in the signature of existing functions in the
Serilalization services, but I’ve added new methods for it in FileUnit
and its subclasses to leave existing implementations untouched.

lib/AST/Module.cpp

@@ -644,6 +644,12 @@ void ModuleDecl::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) const {
   FORWARD(getTopLevelDecls, (Results));
 }
 
+void ModuleDecl::getTopLevelDeclsWhereAttributesMatch(
+              SmallVectorImpl<Decl*> &Results,
+              llvm::function_ref<bool(DeclAttributes)> matchAttributes) const {
+  FORWARD(getTopLevelDeclsWhereAttributesMatch, (Results, matchAttributes));
+}
+
 void SourceFile::getTopLevelDecls(SmallVectorImpl<Decl*> &Results) const {
   Results.append(Decls.begin(), Decls.end());
 }
@@ -1873,6 +1879,22 @@ void *FileUnit::operator new(size_t Bytes, ASTContext &C, unsigned Alignment) {
   return C.Allocate(Bytes, Alignment);
 }
 
+void FileUnit::getTopLevelDeclsWhereAttributesMatch(
+            SmallVectorImpl<Decl*> &Results,
+            llvm::function_ref<bool(DeclAttributes)> matchAttributes) const {
+  auto prevSize = Results.size();
+  getTopLevelDecls(Results);
+
+  // Filter out unwanted decls that were just added to Results.
+  // Note: We could apply this check in all implementations of
+  // getTopLevelDecls instead or in everything that creates a Decl.
+  auto newEnd = std::remove_if(Results.begin() + prevSize, Results.end(),
+                               [&matchAttributes](const Decl *D) -> bool {
+      return !matchAttributes(D->getAttrs());
+    });
+  Results.erase(newEnd, Results.end());
+}
+
 StringRef LoadedFile::getFilename() const {
   return "";
 }