Swift: Support fmtlib for assertions/expectations.

Specifically, this adds custom formatters using `path::operator string()` and `error_code::message()` and dereferences a (non-empty) optional. `fmtlib` provides formatters for these standard library types in `fmt/std.h`, but that file also requires RTTI (which we disable) for `std::exception` so we can't use it without either patching `fmtlib` (which they're open to: https://github.com/fmtlib/fmt/issues/3170) or enabling RTTI (which will require some consideration).

swift/extractor/translators/DeclTranslator.cpp

@@ -253,7 +253,7 @@ void DeclTranslator::fillFunction(const swift::AbstractFunctionDecl& decl,
   entry.name = !decl.hasName() ? "(unnamed function decl)" : constructName(decl.getName());
   entry.body = dispatcher.fetchOptionalLabel(decl.getBody());
   CODEQL_EXPECT_OR(return, decl.hasParameterList(), "Function {} has no parameter list",
-                         entry.name);
+                         *entry.name);
   entry.params = dispatcher.fetchRepeatedLabels(*decl.getParameters());
   auto self = const_cast<swift::ParamDecl* const>(decl.getImplicitSelfDecl());
   entry.self_param = dispatcher.fetchOptionalLabel(self);

swift/logging/Formatters.h

@@ -0,0 +1,20 @@
+#pragma once
+
+// Provides formatters for standard library types to be used with fmtlib.
+// TODO: Patch fmtlib to support using `fmt/std.h` without RTTI
+//       (https://github.com/fmtlib/fmt/issues/3170).
+
+#include <filesystem>
+#include <fmt/format.h>
+#include <string_view>
+
+namespace fmt {
+FMT_FORMAT_AS(std::filesystem::path, std::string);
+}
+
+template <>
+struct fmt::formatter<std::error_code> : fmt::formatter<std::string> {
+  auto format(const std::error_code& e, format_context& ctx) const {
+    return fmt::formatter<std::string>::format(e.message(), ctx);
+  }
+};

swift/logging/SwiftDiagnostics.h

@@ -14,6 +14,8 @@
 #include <fmt/chrono.h>
 #include <nlohmann/json.hpp>
 
+#include "swift/logging/Formatters.h"
+
 namespace codeql {
 
 extern const std::string_view programName;