Port use_lld flag from gyp to gn.

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index b0c5e279dac71041f310d98b6d6f89a7c169c379..71c5e4843092fa9732321964e99cafce9c447c4e 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
+import("//build/toolchain/lld.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Should only be running on Windows.
@@ -57,6 +58,24 @@ template("msvc_toolchain") {
 
   cl = invoker.cl
 
+  if (use_lld) {
+    if (host_os == "win") {
+      lld_link = "lld-link.exe"
+    } else {
+      lld_link = "lld-link"
+    }
+    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
+                         root_build_dir)
+
+    # lld-link includes a replacement for lib.exe that can produce thin
+    # archives and understands bitcode (for lto builds).
+    lib = "$prefix/$lld_link /lib /llvmlibthin"
+    link = "$prefix/$lld_link"
+  } else {
+    lib = "lib.exe"
+    link = "link.exe"
+  }
+
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
@@ -122,7 +141,7 @@ template("msvc_toolchain") {
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
-      command = "$python_path gyp-win-tool link-wrapper $env False lib.exe /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
+      command = "$python_path gyp-win-tool link-wrapper $env False $lib /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
       outputs = [
         # Ignore {{output_extension}} and always use .lib, there's no reason to
@@ -141,7 +160,7 @@ template("msvc_toolchain") {
       libname = "${dllname}.lib"  # e.g. foo.dll.lib
       rspfile = "${dllname}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path gyp-win-tool manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:${dllname}.manifest"
@@ -172,7 +191,7 @@ template("msvc_toolchain") {
       dllname = "{{root_out_dir}}/{{target_output_name}}{{output_extension}}"  # e.g. foo.dll
       rspfile = "${dllname}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
 
       command = link_command
 
@@ -190,7 +209,7 @@ template("msvc_toolchain") {
     tool("link") {
       rspfile = "{{output}}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /OUT:{{output}} /PDB:{{output}}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /OUT:{{output}} /PDB:{{output}}.pdb @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path gyp-win-tool manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:{{output}}.manifest"