Adds a swift tool to the gn mac toolchain and wires up support for Swift source_set targets. Ports FlutterUIPressProxy to Swift.

gn toolchain modifications include:

• Removes //flutter/third_party/libcxx/include and //flutter/third_party/libcxxabi/include from the default include_dirs on all source_set targets, and instead adds them via cflags and cflags_cc so they continue to be picked up by C, C++, Obj-C, Obj-C++ targets. Flutter's custom libc++ library headers should be picked up by C, C++, Objective-C, and Objective-C++ targets, but not Swift targets. Swift code does not use libc++ directly, but many Apple core libraries such as Foundation.framework are implemented in Objective-C/Objective-C++ and directly or indirectly rely on Apple's libc++ implementation in System.framework. Because of this, and because Swift compiles use Clang modules, we should ensure the correct (Apple) libc++ is picked up from Swift targets by ensuring our own libc++ is not.
• Adds a default empty list for swiftflags to source_set targets. This list is used by for Swift source_set targets by the swift tool in the gn toolchain.
• Adds a //build/config/ios:compiler config to engine/src/build/config/ios/BUILD.gn. This sets default swiftflags for iOS targets. This attribute is (only) used by the new swift tool in the toolchain.
• Adds -g to swiftflags for debug (unopt) builds in engine/src/build/config/compiler/BUILD.gn.
• Adds a swift tool to the gn toolchain that invokes swiftc.py to perform compilation and generate a bridging header. This tool is invoked automatically by gn for Swift source_set targets.
• Adds //build/toolchain/apple/swiftc.py, a wrapper script around swiftc in Xcode's Swift toolchain. This script ensures builds are invoked hermetically using a new build cache directory for the emitted swiftmodule, precompiled header, and bridging header, for each compile. The outputs are then copied into the gn output directory. Also adds supporting script //build/toolchain/apple/get_tool_mtime.py and data //build/toolchain/apple/swift_const_gather_protocols.json, both used by swiftc.py.
• Adds a cxx_module tool to the gn toolchain that supports clang module-based compiles. This is used for libc++ transitively via System.framework and others that rely on it.

gn variable modifications include:

• Adds apple_host_toolchain_path variable in engine/src/build/config/apple/apple_sdk.gni. This is set using //build/mac/find_sdk.py to the path of Xcode's XcodeDefault.xctoolchain. This is where Xcode's swiftc tool and some Swift libraries are located. This will be used to create a symlink in the //flutter/prebuilts/SDKs directory to locate Swift tooling during builds.
• Updates enging/src/tools/gn to pre-populate that path as we do with other Xcode SDK path symlinks, to improve performance.
• Adds ios_swift_lib_paths in engine/src/build/config/ios/ios_sdk.gni which holds the path of Swift shared libraries for either iOS physical device or simulator targets. This is within the appropriate iphone or iphonesimulator SDK directory within Xcode.

verify_exported.dart changes:

• Allow exported symbols that match regexp ^_\$s\d+InternalFlutterSwift. To allow Swift symbols to be exported in an Objective-C bridging header, they must be public or open. We only ever use these types internally within the framework and never publish these types in a public header. Like _InternalFlutter Obj-C symbols, we allow InternalFlutterSwift symbols, as they are clearly marked as internal and an end user would need to reverse engineer out their own header to use them.
• Allow Obj-C symbols in the (__DATA,__data) and (__DATA_CONST,__const) in addition to the existing Mach-O sections, so long as it matches the existing criteria.

Finally, this patch adds Swift code to the embedder and embedder tests as proof of concept and to prevent regressions. This ports FlutterUIPressProxy and its subclass FakeUIPressProxy to Swift in order to verify both:

• Objective-C imports of Swift via a bridging header
• Swift imports of Swift via a bridging header

Issue: #144791

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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