[cmake] Fix host tools build in when LLVM_EXPERIMENTAL_TARGETS_TO_BUILD is set

r320413 triggered cmake configure failures when building with 
-DLLVM_OPTIMIZED_TABLEGEN=True and with LLVM_EXPERIMENTAL_TARGETS_TO_BUILD set 
(e.g. to RISCV). This is because that patch moved to passing through 
LLVM_TARGETS_TO_BUILD, and at that point LLVM_EXPERIMENTAL_TARGETS_TO_BUILD 
has been merged in to it. LLVM_EXPERIMENTAL_TARGETS_TO_BUILD must be also be 
passed through to avoid errors like below:

-- Constructing LLVMBuild project information
CMake Error at CMakeLists.txt:682 (message):
  The target `RISCV' does not exist.

      It should be one of


  AArch64;AMDGPU;ARM;BPF;Hexagon;Lanai;Mips;MSP430;NVPTX;PowerPC;Sparc;SystemZ;X86;XCore


-- Configuring incomplete, errors occurred!

See the thread
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20171211/509225.html 
for discussion of this fix.

llvm-svn: 320556

cmake/modules/CrossCompile.cmake

@@ -40,12 +40,15 @@ function(llvm_create_cross_target_internal target_name toolchain buildtype)
   # them to spaces.
   string(REPLACE ";" "$<SEMICOLON>" targets_to_build_arg
          "${LLVM_TARGETS_TO_BUILD}")
+  string(REPLACE ";" "$<SEMICOLON>" experimental_targets_to_build_arg
+         "${LLVM_EXPERIMENTAL_TARGETS_TO_BUILD}")
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
         -DLLVM_TARGETS_TO_BUILD="${targets_to_build_arg}"
+        -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD="${experimental_targets_to_build_arg}"
         ${build_type_flags} ${linker_flag} ${external_clang_dir}
     WORKING_DIRECTORY ${LLVM_${target_name}_BUILD}
     DEPENDS CREATE_LLVM_${target_name}