[Driver][Fuchsia] Pass LTO flags to linker

[phosek]

Summary

Even though we use lld by default for Fuchsia, we use Gold plugin
arguments like all other drivers as lld supports Gold plugin options.

[mcgrathr]

lgtm

clang/lib/Driver/ToolChains/Fuchsia.cpp

if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
     }
   }
 
   Args.AddAllArgs(CmdArgs, options::OPT_L);
   Args.AddAllArgs(CmdArgs, options::OPT_u);
 
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
+  if (D.isUsingLTO()) {
+    assert(!Inputs.empty() && "Must have at least one input.");
+    AddGoldPlugin(ToolChain, Args, CmdArgs, Output, Inputs[0],
+                  D.getLTOMode() == LTOK_Thin);
+  }
+
   addSanitizerRuntimes(ToolChain, Args, CmdArgs);
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     if (Args.hasArg(options::OPT_static))
       CmdArgs.push_back("-Bdynamic");
 

clang/test/Driver/fuchsia.c

 // CHECK-SCUDO-AARCH64: "-pie"
 // CHECK-SCUDO-AARCH64: "{{.*[/\\]}}libclang_rt.scudo-aarch64.so"
 
 // RUN: %clang %s -### --target=x86_64-fuchsia \
 // RUN:     -fsanitize=scudo -fPIC -shared 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-SCUDO-SHARED
 // CHECK-SCUDO-SHARED: "-fsanitize=safe-stack,scudo"
 // CHECK-SCUDO-SHARED: "{{.*[/\\]}}libclang_rt.scudo-x86_64.so"
+
+// RUN: %clang %s -### --target=aarch64-fuchsia \
+// RUN:     -O3 -flto -mcpu=cortex-a53 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-LTO
+// CHECK-LTO: "-plugin-opt=mcpu=cortex-a53"
+// CHECK-LTO: "-plugin-opt=O3"
+
+// RUN: %clang %s -### --target=x86_64-fuchsia \
+// RUN:     -flto=thin -flto-jobs=8 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO
+// CHECK-THINLTO: "-plugin-opt=mcpu=x86-64"
+// CHECK-THINLTO: "-plugin-opt=thinlto"
+// CHECK-THINLTO: "-plugin-opt=jobs=8"