Refactor: shrink diagnostic table with bitfields

Use bitfields to save two bytes from each diagnostic_info.

This commit should not change behavior.

Author: strager

src/diagnostic.cpp

@@ -61,11 +61,14 @@ std::array<char, 5> error_code_to_string(std::uint16_t error_code) noexcept {
 template <class Error>
 class diagnostic_info_builder {
  public:
+  QLJS_WARNING_PUSH
+  QLJS_WARNING_IGNORE_GCC("-Wconversion")
   constexpr explicit diagnostic_info_builder(const char* code_string,
                                              diagnostic_severity sev) {
     this->info_.severity = sev;
     this->info_.code = parse_code_string(code_string);
   }
+  QLJS_WARNING_POP
 
   // Each of Args must be a diagnostic_message_arg_info.
   template <class... Args>

src/quick-lint-js/diagnostic.h

@@ -81,8 +81,8 @@ struct diagnostic_message_info {
 struct diagnostic_info {
   std::array<char, 5> code_string() const noexcept;
 
-  std::uint16_t code;
-  diagnostic_severity severity QLJS_WORK_AROUND_GCC_BUG_105191;
+  std::uint16_t code : 14;
+  diagnostic_severity severity : 2 QLJS_WORK_AROUND_GCC_BUG_105191;
   // If we support more than two infos (i.e. more than one note), the VS Code
   // plugin needs to be updated. See NOTE(multiple notes).
   diagnostic_message_info messages[2];