Fix handling of include prefix

Summary:
D47195241 changed the order of processing includes breaking one corner case. Fix this by always setting the include prefix if it is not empty.

The whole include prefix manipulation is a mess and we should get rid of it but this is a much bigger change. This is a quick fix that restores the old behavior and unbreaks at least one target.

Reviewed By: iahs

Differential Revision: D47340496

fbshipit-source-id: 03a26d1e4bd65ef113364d6e511baace4ff2ce75

thrift/compiler/parse/parse_ast.cc

@@ -1113,17 +1113,17 @@ std::unique_ptr<t_program_bundle> parse_ast(
       program = included_program.get();
       programs->add_program(std::move(included_program));
       program_cache[include_path] = program;
-
-      std::string include_prefix;
-      auto last_slash = include_name.find_last_of("/\\");
-      if (last_slash != std::string::npos) {
-        include_prefix = include_name.substr(0, last_slash);
-      }
-      program->set_include_prefix(include_prefix);
     } else {
       program = it->second;
     }
 
+    std::string include_prefix;
+    auto last_slash = include_name.find_last_of("/\\");
+    if (last_slash != std::string::npos) {
+      include_prefix = include_name.substr(0, last_slash);
+      program->set_include_prefix(include_prefix);
+    }
+
     // Skip already parsed files.
     if (!parsed_paths.insert(include_path).second) {
       return program;