Pull in r227062 from upstream clang trunk (by Renato Golin):

  Allows Clang to use LLVM's fixes-x18 option

  This patch allows clang to have llvm reserve the x18
  platform register on AArch64. FreeBSD will use this in the kernel for
  per-cpu data but has no need to reserve this register in userland so
  will need this flag to reserve it.

  This uses llvm r226664 to allow this register to be reserved.

  Patch by Andrew Turner.

Introduced here: http://svnweb.freebsd.org/changeset/base/277775

Index: tools/clang/include/clang/Driver/Options.td
===================================================================
--- tools/clang/include/clang/Driver/Options.td
+++ tools/clang/include/clang/Driver/Options.td
@@ -1209,6 +1209,8 @@ def mfix_cortex_a53_835769 : Flag<["-"], "mfix-cor
 def mno_fix_cortex_a53_835769 : Flag<["-"], "mno-fix-cortex-a53-835769">,
   Group<m_aarch64_Features_Group>,
   HelpText<"Don't workaround Cortex-A53 erratum 835769 (AArch64 only)">;
+def ffixed_x18 : Flag<["-"], "ffixed-x18">, Group<m_aarch64_Features_Group>,
+  HelpText<"Reserve the x18 register (AArch64 only)">;
 
 def mvsx : Flag<["-"], "mvsx">, Group<m_ppc_Features_Group>;
 def mno_vsx : Flag<["-"], "mno-vsx">, Group<m_ppc_Features_Group>;
Index: tools/clang/lib/Driver/Tools.cpp
===================================================================
--- tools/clang/lib/Driver/Tools.cpp
+++ tools/clang/lib/Driver/Tools.cpp
@@ -958,6 +958,11 @@ void Clang::AddAArch64TargetArgs(const ArgList &Ar
     if (A->getOption().matches(options::OPT_mno_global_merge))
       CmdArgs.push_back("-mno-global-merge");
   }
+
+  if (Args.hasArg(options::OPT_ffixed_x18)) {
+    CmdArgs.push_back("-backend-option");
+    CmdArgs.push_back("-aarch64-reserve-x18");
+  }
 }
 
 // Get CPU and ABI names. They are not independent
Index: tools/clang/test/Driver/aarch64-fixed-x18.c
===================================================================
--- tools/clang/test/Driver/aarch64-fixed-x18.c
+++ tools/clang/test/Driver/aarch64-fixed-x18.c
@@ -0,0 +1,4 @@
+// RUN: %clang -target aarch64-none-gnu -ffixed-x18 -### %s 2> %t
+// RUN: FileCheck --check-prefix=CHECK-FIXED-X18 < %t %s
+
+// CHECK-FIXED-X18: "-backend-option" "-aarch64-reserve-x18"