[clang] [llvm] [CMake] Version Darwin dylib identities (PR #189004)

[Tamir Duberstein via cfe-commits]

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@@ -170,11 +170,12 @@ if(ENABLE_SHARED)
       # implicitly be exported from libclang.
       target_compile_definitions(libclang PRIVATE CLANG_BUILD_STATIC)
   elseif(APPLE)
-    set(LIBCLANG_LINK_FLAGS " -Wl,-compatibility_version -Wl,1")
-    set(LIBCLANG_LINK_FLAGS "${LIBCLANG_LINK_FLAGS} -Wl,-current_version -Wl,${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}")
-
-    set_property(TARGET libclang APPEND_STRING PROPERTY
-                 LINK_FLAGS ${LIBCLANG_LINK_FLAGS})
+    if(LLVM_VERSIONED_DYLIB_NAME_ON_DARWIN)
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tamird wrote:

> I think there is still a version number on the dylib right here right? It is just not equal to llvm version.
> 
> macOS toolchain has always been shipping with no version and new toolchain cannot have a version on it, otherwise, it will break compatibility with old tools. Let me know if I misunderstood the code.

I think this is not correct; even before this PR libclang and libLTO had explicit version numbers, they were just doing it manually using `-compatibility_version` and `-current_version`. The second commit in this PR kept that behavior but changed the implementation to use `MACHO_COMPATIBILITY_VERSION` and `MACHO_CURRENT_VERSION`.

https://github.com/llvm/llvm-project/pull/189004