[Apple] Fix `gn gen` when using swift and no_stamp_files

When using swift source files, the outputs cannot be derived from
the sources (due to the "whole module optimisation" model). This
caused `gn gen` to fail an assertion when using no_stamp_files
and whole module optimisation.

Without this change, generating the build for the sample project
(in //examples/ios) would fail like this:

  $ ../../out/gn gen out
  [...:FATAL:ninja_c_binary_target_writer.cc(539)]
  Check failed: swiftmodule->has_dependency_output().

  [...:FATAL:ninja_target_writer.cc(626)]
  Check failed: files.empty().

  [...:FATAL:ninja_c_binary_target_writer.cc(539)]
  Check failed: swiftmodule->has_dependency_output().

Fix this by ensuring HasRealInputs() returns true when the source_set
contains any swift source files.

Bug: none
Change-Id: Icb748717e1bcb301e3e32fdbba5293f2d09ad69d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/19580
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

src/gn/target.cc

diff --git a/src/gn/target.cc b/src/gn/target.cc
index 6cb5572..1ea1298 100644
--- a/src/gn/target.cc
+++ b/src/gn/target.cc
@@ -931,6 +931,11 @@
   std::vector<OutputFile> tool_outputs;
   return std::any_of(
       sources().begin(), sources().end(), [&, this](const auto& source) {
+        // Swift files always results in output files, but the name cannot
+        // be derived from the source file via GetOutputFilesForSource(...).
+        if (source.GetType() == SourceFile::SOURCE_SWIFT) {
+          return true;
+        }
         const char* tool_name = Tool::kToolNone;
         return GetOutputFilesForSource(source, &tool_name, &tool_outputs);
       });