Set CLANG_TABLEGEN when cross compiling clang

When cross compiling rustc with `llvm.clang = true`, CLANG_TABLEGEN
has to be set to the host clang-tblgen executable to build clang.

Author: topjohnwu

Diff for: src/bootstrap/native.rs

@@ -417,11 +417,20 @@ impl Step for Llvm {
             //        actually exists most of the time in normal installs of LLVM.
             let host_bin = builder.llvm_out(builder.config.build).join("bin");
             cfg.define("LLVM_TABLEGEN", host_bin.join("llvm-tblgen").with_extension(EXE_EXTENSION));
+            // LLVM_NM is required for cross compiling using MSVC
             cfg.define("LLVM_NM", host_bin.join("llvm-nm").with_extension(EXE_EXTENSION));
             cfg.define(
                 "LLVM_CONFIG_PATH",
                 host_bin.join("llvm-config").with_extension(EXE_EXTENSION),
             );
+            if builder.config.llvm_clang {
+                let build_bin = builder.llvm_out(builder.config.build).join("build").join("bin");
+                let clang_tblgen = build_bin.join("clang-tblgen").with_extension(EXE_EXTENSION);
+                if !clang_tblgen.exists() {
+                    panic!("unable to find {}", clang_tblgen.display());
+                }
+                cfg.define("CLANG_TABLEGEN", clang_tblgen);
+            }
         }
 
         if let Some(ref suffix) = builder.config.llvm_version_suffix {