[gn build] Support building with ThinLTO

llvm/utils/gn/build/BUILD.gn

config("compiler_defaults") {
   }
 
   if (use_tsan) {
     assert(is_clang && current_os == "linux",
            "tsan only supported on Linux/Clang")
     cflags += [ "-fsanitize=thread" ]
     ldflags += [ "-fsanitize=thread" ]
   }
+
+  if (use_thinlto) {
+    assert(is_clang, "ThinLTO only supported on Clang")
+
+    lto_opt_level = 2
+
+    cflags += [ "-flto=thin" ]
+
+    if (host_os == "win") {
+      ldflags += [
+        "/opt:lldlto=" + lto_opt_level,
+        "/opt:lldltojobs=" + max_jobs_per_lto_link,
+      ]
+    } else {
+      ldflags += [
+        "-flto=thin",
+        "-Wl,--thinlto-jobs=" + max_jobs_per_lto_link,
+        "-Wl,--lto-O" + lto_opt_level,
+      ]
+    }
+  }
 }
 
 config("no_exceptions") {
   if (host_os != "win") {
     cflags_cc = [ "-fno-exceptions" ]
   }
 }
 

llvm/utils/gn/build/buildflags.gni

 declare_args() {
   # Whether to build with debug information.
   is_debug = false
 
   # Whether to build with tsan.
   use_tsan = false
 
   # Whether to build with ubsan.
   use_ubsan = false
 
   # Whether to build with asan.
   use_asan = false
-}
-
-# args that depend on other args must live in a later declare_args() block.
-declare_args() {
-  # Whether to build with optimizations.
-  is_optimized = !is_debug
 
   # Whether to enable assertions.
   llvm_enable_assertions = true
 
   # Whether to enable expensive checks.
   llvm_enable_expensive_checks = false
+
+  # Whether to build with ThinLTO.
+  use_thinlto = false
+
+  # Max jobs per ThinLTO link.
+  max_jobs_per_lto_link = 8
+}
+
+# args that depend on other args must live in a later declare_args() block.
+declare_args() {
+  # Whether to build with optimizations.
+  is_optimized = !is_debug
 }