[mips] Add support for -m(no-)extern-data.

[sdardis]

Summary

Add support for -m(no-)extern-data when using -mgpopt in the driver. It is
enabled by default in the backend.

[atanasyan]

LGTM

include/clang/Driver/Options.td

HelpText<"Use GP relative accesses for symbols known to be in a small"
            " data section (MIPS)">;
 def mno_gpopt : Flag<["-"], "mno-gpopt">, Group<m_Group>,
   HelpText<"Do not use GP relative accesses for symbols known to be in a small"
            " data section (MIPS)">;
 def mlocal_sdata : Flag<["-"], "mlocal-sdata">, Group<m_Group>,
   HelpText<"Extend the -G behaviour to object local data (MIPS)">;
 def mno_local_sdata : Flag<["-"], "mno-local-sdata">, Group<m_Group>,
   HelpText<"Do not extend the -G behaviour to object local data (MIPS)">;
+def mextern_sdata : Flag<["-"], "mextern-sdata">, Group<m_Group>,
+  HelpText<"Assume that externally defined data is in the small data if it"
+           " meets the -G <size> threshold (MIPS)">;
+def mno_extern_sdata : Flag<["-"], "mno-extern-sdata">, Group<m_Group>,
+  HelpText<"Do not assume that externally defined data is in the small data if"
+           " it meets the -G <size> threshold (MIPS)">;
 def mnan_EQ : Joined<["-"], "mnan=">, Group<m_Group>;
 def mabicalls : Flag<["-"], "mabicalls">, Group<m_Group>,
   HelpText<"Enable SVR4-style position-independent code (Mips only)">;
 def mno_abicalls : Flag<["-"], "mno-abicalls">, Group<m_Group>,
   HelpText<"Disable SVR4-style position-independent code (Mips only)">;
 def mips1 : Flag<["-"], "mips1">,
   Alias<march_EQ>, AliasArgs<["mips1"]>,
   HelpText<"Equivalent to -march=mips1">, Flags<[HelpHidden]>;

lib/Driver/ToolChains/Clang.cpp

if (Arg *A = Args.getLastArg(options::OPT_G)) {
     CmdArgs.push_back(Args.MakeArgString("-mips-ssection-threshold=" + v));
     A->claim();
   }
 
   Arg *GPOpt = Args.getLastArg(options::OPT_mgpopt, options::OPT_mno_gpopt);
   Arg *ABICalls = Args.getLastArg(options::OPT_mabicalls, options::OPT_mno_abicalls);
   Arg *LocalSData =
       Args.getLastArg(options::OPT_mlocal_sdata, options::OPT_mno_local_sdata);
+  Arg *ExternSData = Args.getLastArg(options::OPT_mextern_sdata, options::OPT_mno_extern_sdata);
   // -mabicalls is the default for many MIPS environments, even with -fno-pic.
   // -mgpopt is the default for static, -fno-pic environments but these two
   // options conflict. We want to be certain that -mno-abicalls -mgpopt is
   // the only case where -mllvm -mgpopt is passed.
   // NOTE: We need a warning here or in the backend to warn when -mgpopt is
   //       passed explicitly when compiling something with -mabicalls
   //       (implictly) in affect. Currently the warning is in the backend.
   bool WantABICalls =
if (LocalSData) {
       CmdArgs.push_back("-mllvm");
       if (LocalSData->getOption().matches(options::OPT_mlocal_sdata)) {
         CmdArgs.push_back("-mlocal-sdata=1");
       } else {
         CmdArgs.push_back("-mlocal-sdata=0");
       }
       LocalSData->claim();
     }
+
+    if (ExternSData) {
+      CmdArgs.push_back("-mllvm");
+      if (ExternSData->getOption().matches(options::OPT_mextern_sdata)) {
+        CmdArgs.push_back("-mextern-sdata=1");
+      } else {
+        CmdArgs.push_back("-mextern-sdata=0");
+      }
+      ExternSData->claim();
+    }
   }
 
   if (Arg *A = Args.getLastArg(options::OPT_mcompact_branches_EQ)) {
     StringRef Val = StringRef(A->getValue());
     if (mips::hasCompactBranches(CPUName)) {
       if (Val == "never" || Val == "always" || Val == "optimal") {
         CmdArgs.push_back("-mllvm");
         CmdArgs.push_back(Args.MakeArgString("-mips-compact-branches=" + Val));

test/Driver/mips-features.c

 // RUN:   | FileCheck --check-prefix=CHECK-MNOLOCALSDATA %s
 // CHECK-MNOLOCALSDATA: "-mllvm" "-mlocal-sdata=0"
 //
 // -mgpopt -mno-abicalls
 // RUN: %clang -target mips-linux-gnu -### -c %s -mno-abicalls -mgpopt 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-MLOCALSDATADEF %s
 // CHECK-MLOCALSDATADEF-NOT: "-mllvm" "-mlocal-sdata"
 //
+// -mno-abicalls -mgpopt -mextern-sdata
+// RUN: %clang -target mips-linux-gnu -### -c %s -mno-abicalls -mgpopt -mno-extern-sdata -mextern-sdata 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-MEXTERNSDATA %s
+// CHECK-MEXTERNSDATA: "-mllvm" "-mextern-sdata=1"
+//
+// -mno-abicalls -mgpopt -mno-extern-sdata
+// RUN: %clang -target mips-linux-gnu -### -c %s -mno-abicalls -mgpopt -mextern-sdata -mno-extern-sdata 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-MNOEXTERNSDATA %s
+// CHECK-MNOEXTERNSDATA: "-mllvm" "-mextern-sdata=0"
+//
+// -mno-abicalls -mgpopt
+// RUN: %clang -target mips-linux-gnu -### -c %s -mno-abicalls -mgpopt 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-MEXTERNSDATADEF %s
+// CHECK-MEXTERNSDATADEF-NOT: "-mllvm" "-mextern-sdata"
+//
 // -mips16
 // RUN: %clang -target mips-linux-gnu -### -c %s \
 // RUN:     -mno-mips16 -mips16 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS16 %s
 // CHECK-MIPS16: "-target-feature" "+mips16"
 //
 // -mno-mips16
 // RUN: %clang -target mips-linux-gnu -### -c %s \