WiX: package up DocC conditionally

[compnerd]

Add support to package up DocC in the windows distribution. This is the last large component that was missing. At this point, the toolchain distribution is generally speaking complete with:

• Preprocessor (clang)
• Assembler (IAS)
• C/C++/Swift Compiler (clang/clang++/swiftc)
• Linker (lld)
• Binary Utilities (LLVM)
• Debugger (LLDB)
• REPL
• Package Manager (SwiftPM)
• Runtime Inspection Tools (swift-inspect)
• LSP (sourcekit-lsp)
• Code Formatter (swift-format)
• Documentation Tools (DocC)
• Additional IDE integration Tools (lldb-vscode)
• Redistributables (MSMs)
• Runtime (host-only)
• SDKs (AMD64, ARM64, X86)

[compnerd]

Tested at https://ci-external.swift.org/job/swift-PR-build-toolchain-windows/1066/