gn build: Create a variable for the host toolchain and start using it in the tblgen template.

[thakis]

lgtm assuming that explicitly mentioning the default toolchain in a label is a no-op.

llvm/utils/gn/build/BUILDCONFIG.gn

 set_defaults("shared_library") {
   configs = shared_binary_target_configs
 }
 set_defaults("source_set") {
   configs = shared_binary_target_configs
 }
 
 if (host_os == "win") {
-  set_default_toolchain("//llvm/utils/gn/build/toolchain:win")
+  host_toolchain = "//llvm/utils/gn/build/toolchain:win"
 } else {
-  set_default_toolchain("//llvm/utils/gn/build/toolchain:unix")
+  host_toolchain = "//llvm/utils/gn/build/toolchain:unix"
 }
+
+set_default_toolchain(host_toolchain)

llvm/utils/gn/secondary/clang/utils/TableGen/clang_tablegen.gni

tablegen(target_name) {
     forward_variables_from(invoker,
                            [
                              "args",
                              "output_name",
                              "td_file",
                              "visibility",
                            ])
 
-    # FIXME: In cross builds, this should depend on the host binary.
     tblgen_target = "//clang/utils/TableGen:clang-tblgen"
   }
 }

llvm/utils/gn/secondary/llvm/utils/TableGen/tablegen.gni

template("tablegen") {
   }
 
   action(target_name) {
     forward_variables_from(invoker, [ "visibility" ])
 
     if (defined(invoker.tblgen_target)) {
       tblgen_target = invoker.tblgen_target
     } else {
-      # FIXME: In cross builds, this should depend on the host binary.
       tblgen_target = "//llvm/utils/TableGen:llvm-tblgen"
     }
+    tblgen_target += "($host_toolchain)"
     tblgen_executable = get_label_info(tblgen_target, "root_out_dir") +
                         "/bin/" + get_label_info(tblgen_target, "name")
     deps = [
       tblgen_target,
     ]
     if (defined(invoker.td_file)) {
       td_file = invoker.td_file
     } else {