CMake: Make libxml2 show up in --system-libs (PR36660)

[hans]

Summary

lib/WindowsManifest/CMakeLists.txt adds it to LLVM_SYSTEM_LIBS on that
target, but it was never getting picked up in
tools/llvm-config/CMakeLists.txt.

tools/llvm-config/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS support)
 
 set(BUILDVARIABLES_SRCPATH ${CMAKE_CURRENT_SOURCE_DIR}/BuildVariables.inc.in)
 set(BUILDVARIABLES_OBJPATH ${CMAKE_CURRENT_BINARY_DIR}/BuildVariables.inc)
 
 # Add the llvm-config tool.
 add_llvm_tool(llvm-config
   llvm-config.cpp
   )
 
 # Compute the substitution values for various items.
-get_property(LLVM_SYSTEM_LIBS_LIST TARGET LLVMSupport PROPERTY LLVM_SYSTEM_LIBS)
-foreach(l ${LLVM_SYSTEM_LIBS_LIST})
+get_property(SUPPORT_SYSTEM_LIBS TARGET LLVMSupport PROPERTY LLVM_SYSTEM_LIBS)
+get_property(WINDOWSMANIFEST_SYSTEM_LIBS TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS)
+foreach(l ${SUPPORT_SYSTEM_LIBS} ${WINDOWSMANIFEST_SYSTEM_LIBS})
   if(MSVC)
     set(SYSTEM_LIBS ${SYSTEM_LIBS} "${l}.lib")
   else()
     if (l MATCHES "^-")
       # If it's an option, pass it without changes.
       set(SYSTEM_LIBS ${SYSTEM_LIBS} "${l}")
     else()
       # Otherwise assume it's a library name we need to link with.