[llvm] [cmake] Set LLVM_LINKER_IS_MOLD variable when using Mold linker

Surrounding code for other linkers already sets similar variables
(LLVM_LINKER_IS_GNULD, LLVM_LINKER_IS_GOLD, LLVM_LINKER_IS_LLD etc.).

--- /llvm/cmake/modules/AddLLVM.cmake
+++ /llvm/cmake/modules/AddLLVM.cmake
@@ -212,6 +212,7 @@
   else()
     if("${stdout}" MATCHES "^mold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
+      set(LLVM_LINKER_IS_MOLD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: mold")
     elseif("${stdout}" MATCHES "GNU gold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")