Temporarily revert "build: avoid hardcoding the libxml2 library name"

as it breaks uses of llvm-config --system-libs and the follow-on commit
"build: avoid cached literals being linked against"

This reverts commits 340e7c0b77 and
340e7c0b77.

llvm/cmake/config-ix.cmake

@@ -166,6 +166,7 @@ if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
         else()
           include_directories(${LIBXML2_INCLUDE_DIR})
         endif()
+        set(LIBXML2_LIBS "xml2")
       endif()
     endif()
   endif()

llvm/lib/WindowsManifest/CMakeLists.txt

@@ -1,12 +1,18 @@
+set(system_libs)
+if( CMAKE_HOST_UNIX )
+  if( LLVM_LIBXML2_ENABLED )
+    set(system_libs ${system_libs} ${LIBXML2_LIBS})
+  endif()
+endif()
+
 add_llvm_component_library(LLVMWindowsManifest
   WindowsManifestMerger.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/WindowsManifest
-  ${Backtrace_INCLUDE_DIRS})
+  ${Backtrace_INCLUDE_DIRS}
 
-if(LIBXML2_LIBRARIES)
-  target_link_libraries(LLVMWindowsManifest PUBLIC ${LIBXML2_LIBRARIES})
-  set_property(TARGET LLVMWindowsManifest PROPERTY
-    LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})
-endif()
+  LINK_LIBS ${system_libs}
+  )
+
+set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")