CMake: Fix map_components macro with LINK_LLVM_DYLIB

[jschueller]

Summary

Tries to address https://bugs.llvm.org/show_bug.cgi?id=35245

The cmake macro llvm_map_components_to_libnames is advertised in llvm/docs/CMake.rst
But it does not work on archlinux, where LLVM is compiled with LLVM_LINK_LLVM_DYLIB=ON
In that case there are no targets LLVMCore, LLVMEngine, etc..., instead we must link to the "LLVM" target

Else it errors with these kind of errors:
/usr/bin/ld: cannot find -lLLVMCore: No such file or directory

Is even llvm_map_components_to_libnames still relevant these days ?

[jschueller]

This comment was removed by jschueller.

llvm/cmake/modules/LLVM-Config.cmake

if(NOT LLVM_TARGETS_CONFIGURED)
     foreach(c ${link_components})
       is_llvm_target_specifier("${c}" iltl_result ALL_TARGETS)
       if(iltl_result)
         message(FATAL_ERROR "Specified target library before target registration is complete.")
       endif()
     endforeach()
   endif()
 
+  # all symbols come from libLLVM library
+  # https://github.com/llvm/llvm-project/issues/34593
+  if(LLVM_LINK_LLVM_DYLIB)
+    set(${out_libs} LLVM PARENT_SCOPE)
+    return ()
+  endif ()
+
   # Expand some keywords:
   if(engine IN_LIST link_components)
     if(${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_TO_BUILD AND
        ${LLVM_NATIVE_ARCH} IN_LIST LLVM_TARGETS_WITH_JIT)
       list(APPEND link_components "jit")
       list(APPEND link_components "native")
     else()
       list(APPEND link_components "interpreter")