Support LLD linker for Darwin (#286)

Users can use with `--linkopt=-fuse-ld=ld64.lld` flag.

Eventually, we should make this the default. But only after we hear from
some users that it works for their projects. This PR will make it easy
for them to test.

Author: siddharthab

tests/scripts/run_tests.sh

@@ -42,6 +42,7 @@ set -x
 test_args=(
   "--extra_toolchains=${toolchain_name}"
   "--copt=-v"
+  "--linkopt=-Wl,-v"
   "--linkopt=-Wl,-t"
 )
 

toolchain/BUILD.llvm_repo

@@ -34,9 +34,7 @@ filegroup(
 
 filegroup(
     name = "ld",
-    srcs = [
-        "bin/ld.lld",
-    ],
+    srcs = glob(["bin/ld.lld", "bin/ld64.lld"]),
 )
 
 filegroup(

toolchain/osx_cc_wrapper.sh.tpl

@@ -56,12 +56,14 @@ function parse_option() {
 
 if [[ -f %{toolchain_path_prefix}bin/clang ]]; then
   execroot_path=""
+  execroot_abs_path="${PWD}/"
 elif [[ ${BASH_SOURCE[0]} == "/"* ]]; then
   # Some consumers of `CcToolchainConfigInfo` (e.g. `cmake` from rules_foreign_cc)
   # change CWD and call $CC (this script) with its absolute path.
   # For cases like this, we'll try to find `clang` through an absolute path.
   # This script is at _execroot_/external/_repo_name_/bin/cc_wrapper.sh
   execroot_path="${BASH_SOURCE[0]%/*/*/*/*}/"
+  execroot_abs_path="$(cd "${execroot_path}" && pwd -P)/"
 else
   echo >&2 "ERROR: could not find clang; PWD=\"${PWD}\"; PATH=\"${PATH}\"."
   exit 5
@@ -71,6 +73,8 @@ function sanitize_option() {
   local -r opt=$1
   if [[ ${opt} == */cc_wrapper.sh ]]; then
     printf "%s" "${execroot_path}%{toolchain_path_prefix}bin/clang"
+  elif [[ ${opt} == "-fuse-ld=ld64.lld" ]]; then
+    echo "-fuse-ld=${execroot_abs_path}%{toolchain_path_prefix}bin/ld64.lld"
   elif [[ ${opt} =~ ^-fsanitize-(ignore|black)list=[^/] ]]; then
     # shellcheck disable=SC2206
     parts=(${opt/=/ }) # Split flag name and value into array.