[cmake] Include LLVM_LIBXML2_ENABLED in LLVMConfig.cmake, PR36006

[mgorny]

Summary

Include the LLVM_LIBXML2_ENABLED cache variable in LLVMConfig.cmake
in order to make it available for other LLVM packages to query. This
is necessary to fix stand-alone testing of LLD.

[hans]

lgtm

llvm/trunk/cmake/modules/LLVMConfig.cmake.in

 set(LLVM_ENABLE_RTTI @LLVM_ENABLE_RTTI@)
 
 set(LLVM_ENABLE_TERMINFO @LLVM_ENABLE_TERMINFO@)
 
 set(LLVM_ENABLE_THREADS @LLVM_ENABLE_THREADS@)
 
 set(LLVM_ENABLE_ZLIB @LLVM_ENABLE_ZLIB@)
 
+set(LLVM_LIBXML2_ENABLED @LLVM_LIBXML2_ENABLED@)
+
 set(LLVM_ENABLE_DIA_SDK @LLVM_ENABLE_DIA_SDK@)
 
 set(LLVM_NATIVE_ARCH @LLVM_NATIVE_ARCH@)
 
 set(LLVM_ENABLE_PIC @LLVM_ENABLE_PIC@)
 
 set(LLVM_BUILD_32_BITS @LLVM_BUILD_32_BITS@)