build: use add_llvm_tool_symlink

[compnerd]

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

Rather than invoke the create_symlink command manually which wont work on
Windows, use the add_llvm_tool_symlink.

[compnerd]

@swift-ci please test

[compnerd]

CC: @llvm-beanz

[hughbe]

LGTM, will cause merge conflicts with my Windows port (I won't be available to contribute until Wednesday)... up to you in deciding which order to merge

[hughbe]

Actually... just realised this is slightly broken - Windows files end in "exe" but this doesn't account for that?

[compnerd]

That existed prior to this and the add_llvm_tool_symlink uses the ${CMAKE_EXECUTABLE_SUFFIX} which should add the .exe for you :-).

[hughbe]

Awesome good to know - what about other symlinks for python files and the like? Does LLVM have an equivalent

[llvm-beanz]

@compnerd I would make add_swift_tool_symlink basically the same as add_clang_symlink is in Clang. Since you're using it the same way every time.

@hughbe LLVM doesn't yet have a more-generic equivalent, but I need to make one for LLDB, so it will.

[compnerd]

@llvm-beanz sure, will do a follow up commit soon.

[compnerd]

@llvm-beanz created PR #6053 for the suggestion.

@hughbe we should be able to simplify the changes that you had for Windows support now as we dont need to do all the craziness for the symlinking ourselves.

[hughbe]

@compnerd some of it, yes: symlinking clang header directories and swift-api-dump.py will need to be changed when @llvm-beanz adds the macro he suggested