[gn build] allow setting clang_base_path to a source-absolute path

[thakis]

Summary

With this, you can set clang_base_path = "//out/gn1" in out/gn2/args.gn and the build in out/gn2 will use clang and lld from out/gn1.

Setting clang_base_path to an absolute path (with e.g. clang_base_path = getenv("HOME") + "/src/...") should behave as before.

llvm/utils/gn/build/toolchain/BUILD.gn

toolchain(target_name) {
     not_needed("*")
 
     forward_variables_from(invoker, "*")
 
     cc = "cc"
     cxx = "c++"
 
     if (clang_base_path != "") {
-      cc = "$clang_base_path/bin/clang"
-      cxx = "$clang_base_path/bin/clang++"
+      cc = rebase_path(clang_base_path, root_build_dir) + "/bin/clang"
+      cxx = rebase_path(clang_base_path, root_build_dir) + "/bin/clang++"
     }
 
     ld = cxx  # Don't use goma wrapper for linking.
     if (use_goma) {
       cc = "$goma_dir/gomacc $cc"
       cxx = "$goma_dir/gomacc $cxx"
     }
 

 
 # This template defines a toolchain that uses just-built clang and lld
 # as compiler and linker.
 template("stage2_unix_toolchain") {
   unix_toolchain(target_name) {
     toolchain_args = {
       forward_variables_from(invoker.toolchain_args, "*")
 
-      clang_base_path = "."
+      clang_base_path = root_build_dir
       use_goma = false
     }
 
     deps = [
       "//:clang($host_toolchain)",
       "//:lld($host_toolchain)",
     ]
     if (current_os != "ios" && current_os != "mac") {
if (host_os == "mac") {
   }
 }
 
 toolchain("win") {
   cl = "cl"
   link = "link"
 
   if (clang_base_path != "") {
-    cl = "$clang_base_path/bin/clang-cl"
+    cl = rebase_path(clang_base_path, root_build_dir) + "/bin/clang-cl"
     if (use_lld) {
-      link = "$clang_base_path/bin/lld-link"
+      link = rebase_path(clang_base_path, root_build_dir) + "/bin/lld-link"
     }
   }
 
   if (use_goma) {
     cl = "$goma_dir/gomacc $cl"
   }
 
   tool("cc") {