Simplify linking with libraries that contain Swift code

[rolfbjarne]

Currently developers need to add custom linker flags if any of their third-party libraries need to link with swift.

Example code:

<Target Name="LinkWithSwift" DependsOnTargets="_ParseBundlerArguments;_DetectSdkLocations" BeforeTargets="_LinkNativeExecutable">
	<PropertyGroup>
		<_SwiftPlatform Condition="$(RuntimeIdentifier.StartsWith('iossimulator-'))">iphonesimulator</_SwiftPlatform>
		<_SwiftPlatform Condition="$(RuntimeIdentifier.StartsWith('ios-'))">iphoneos</_SwiftPlatform>
	</PropertyGroup>
	<ItemGroup>
		<_CustomLinkFlags Include="-L" />
		<_CustomLinkFlags Include="/usr/lib/swift" />
		<_CustomLinkFlags Include="-L" />
		<_CustomLinkFlags Include="$(_SdkDevPath)/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/$(_SwiftPlatform)" />
		<_CustomLinkFlags Include="-Wl,-rpath" />
		<_CustomLinkFlags Include="-Wl,/usr/lib/swift" />
	</ItemGroup>
</Target>

Ref: #18774 (comment)

We should consider adding something like this into our product, to make it easier for everyone referencing libraries that use swift.