chore: add example of SWIFT_PACKAGE defines

[luispadron]

Showcases an issue with SWIFT_PACKAGE defines when consuming a Swift Package like: https://github.com/luispadron/swift-package-defines-example

The package above builds with swift build but fails when building through bazel with:

bazel build @swiftpkg_swift_package_defines_example//:FooSwift.rspm

ERROR: /private/var/tmp/_bazel_luispadron/5af19db7d5e52089e6b3b651a88eb3e7/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/BUILD.bazel:34:14: Compiling Swift module @@rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example//:FooSwift.rspm failed: (Exit 1): worker failed: error executing SwiftCompile command (from target @@rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example//:FooSwift.rspm) 
  (cd /private/var/tmp/_bazel_luispadron/5af19db7d5e52089e6b3b651a88eb3e7/execroot/_main && \
  exec env - \
    APPLE_SDK_PLATFORM=MacOSX \
    APPLE_SDK_VERSION_OVERRIDE=15.0 \
    PATH=/bin:/usr/bin:/usr/local/bin \
    XCODE_VERSION_OVERRIDE=16.0.0.16A242d \
  bazel-out/darwin_arm64-opt-exec-ST-d57f47055a04/bin/external/rules_swift~/tools/worker/worker swiftc @bazel-out/darwin_arm64-fastbuild/bin/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/FooSwift.swiftmodule-0.params)
# Configuration: c0e955b657088cfeaf65914df4e2f29ac5886d3e2dcf08bafe9d3d979080c98e
# Execution platform: @@platforms//host:host
external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/Sources/FooSwift/Foo.swift:5:24: error: cannot find 'foo' in scope
3 | public func fooSwift() {
4 |     print("FooSwift")
5 |     print("ObjC foo: \(foo())")
  |                        `- error: cannot find 'foo' in scope
6 | }
7 | 

#1260 allows the same build to pass

[cgrindel]

I dumped the subcommands while compiling this example. The following is the compilation command for Foo.m:

external/apple_support~~apple_cc_configure_extension~local_config_apple_cc/wrapped_clang \
-target \
x86_64-apple-macosx13.0 \
'-D_FORTIFY_SOURCE=1' \
-fstack-protector \
-fcolor-diagnostics \
-Wall \
-Wthread-safety \
-Wself-assign \
-fno-omit-frame-pointer \
-O0 \
-DDEBUG \
'-fdebug-prefix-map=__BAZEL_EXECUTION_ROOT__=.' \
'-fdebug-prefix-map=__BAZEL_XCODE_DEVELOPER_DIR__=/PLACEHOLDER_DEVELOPER_DIR' \
'-Werror=incompatible-sysroot' \
-Wshorten-64-to-32 \
-Wbool-conversion \
-Wconstant-conversion \
-Wduplicate-method-match \
-Wempty-body \
-Wenum-conversion \
-Wint-conversion \
-Wunreachable-code \
-Wmismatched-return-types \
-Wundeclared-selector \
-Wuninitialized \
-Wunused-function \
-Wunused-variable \
-iquote \
external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example \
-iquote \
bazel-out/darwin_x86_64-fastbuild/bin/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example \
-Iexternal/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/Sources/Foo/include \
-Ibazel-out/darwin_x86_64-fastbuild/bin/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/Sources/Foo/include \
-MD \
-MF \
bazel-out/darwin_x86_64-fastbuild/bin/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/_objs/Foo.rspm/arc/Foo.d \
-DOS_MACOSX \
-fno-autolink \
-isysroot \
__BAZEL_XCODE_SDKROOT__ \
-F__BAZEL_XCODE_SDKROOT__/System/Library/Frameworks \
-F__BAZEL_XCODE_DEVELOPER_DIR__/Platforms/MacOSX.platform/Developer/Library/Frameworks \
-fobjc-arc \
-fblocks \
-fobjc-arc \
-fPIC \
'-fmodule-name=Foo' \
'-DSWIFT_PACKAGE=1' \
-fmodules \
'-fmodules-cache-path=bazel-out/darwin_x86_64-fastbuild/bin/_objc_module_cache' \
-O0 \
'-DDEBUG=1' \
-c \
external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/Sources/Foo/Foo.m \
-o \
bazel-out/darwin_x86_64-fastbuild/bin/external/rules_swift_package_manager~~swift_deps~swiftpkg_swift_package_defines_example/_objs/Foo.rspm/arc/Foo.o)

The SWIFT_PACKAGE macro is being specified.

[cgrindel]

I confirmed that Foo.o has the _foo symbol.

$ nm _objs/Foo.rspm/arc/Foo.o
0000000000000000 T _foo

I think the issue is related to how Swift processes the header files. Swift independently reads the header files via the include directories. Hence, the need to pass the macro to clang via Swift.