[gn build] Generalize use_goma to compiler_wrapper

[aeubanks]

Summary

e.g. ccache is much faster than goma if everything is already cached.

Also since goma is going away.

Users will need to specify compiler_wrapper = $prev_goma_dir/gomacc for the same behavior.

[aeubanks]

remove goma gn args

[aeubanks]

https://github.com/llvm/llvm-project/pull/72757

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

 import("//llvm/utils/gn/build/toolchain/compiler.gni")
 
 declare_args() {
   # If is_goma is true, the location of the goma client install.
   # Set this to the output of `goma_ctl goma_dir`.
   goma_dir = ""
+
+  # If set, prepends this to compile action command lines (e.g. `"ccache"`).
+  compiler_wrapper = ""
 }
 
 assert(!use_goma || goma_dir != "",
        "set `goma_dir` to the output of `goma_ctl goma_dir` in your args.gn")
 
+assert(!(use_goma && compiler_wrapper != ""), "cannot use both goma and a compiler launcher")
+
 unix_copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 
 template("unix_toolchain") {
   toolchain(target_name) {
     # https://groups.google.com/a/chromium.org/g/gn-dev/c/F_lv5T-tNDM
     forward_variables_from(invoker.toolchain_args, "*")
     not_needed("*")
 
     forward_variables_from(invoker, "*")
 
     cc = "cc"
     cxx = "c++"
 
     if (clang_base_path != "") {
       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 $cxx"
+    } else if (compiler_wrapper != "") {
+      cc = "$compiler_wrapper $cc"
+      cxx = "$compiler_wrapper $cxx"
     }
 
     tool("cc") {
       depfile = "{{output}}.d"
       command = "$cc -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
       depsformat = "gcc"
       description = "CC {{output}}"
       outputs = [ "{{source_out_dir}}/{{label_name}}.{{source_name_part}}.o" ]