[RISCV] Avoid save-restore target feature warning

Summary:
LLVM issues a warning if passed unknown target features. Neither I nor
@asb noticed this until after https://reviews.llvm.org/D63498 landed.

This patch stops passing the (unknown) "save-restore" target feature to
the LLVM backend, but continues to emit a warning if a driver asks for
`-msave-restore`. The default of assuming `-mno-save-restore` (and
emitting no warnings) remains.

Reviewers: asb

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, Jim, cfe-commits, asb

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64008

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@364777 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/ToolChains/Arch/RISCV.cpp

@@ -358,14 +358,12 @@ void riscv::getRISCVTargetFeatures(const Driver &D, const ArgList &Args,
   else
     Features.push_back("-relax");
 
-  // -mno-save-restore is default, unless -msave-restore is specified.
+  // GCC Compatibility: -mno-save-restore is default, unless -msave-restore is
+  // specified...
   if (Args.hasFlag(options::OPT_msave_restore, options::OPT_mno_save_restore, false)) {
-    Features.push_back("+save-restore");
-    // ... but we don't yet support +save-restore, so issue a warning.
+    // ... but we don't support -msave-restore, so issue a warning.
     D.Diag(diag::warn_drv_clang_unsupported)
       << Args.getLastArg(options::OPT_msave_restore)->getAsString(Args);
-  } else {
-    Features.push_back("-save-restore");
   }
 
   // Now add any that the user explicitly requested on the command line,

test/Driver/riscv-features.c

@@ -17,9 +17,5 @@
 // RUN: %clang -target riscv32-unknown-elf -### %s -mno-save-restore 2>&1 | FileCheck %s -check-prefix=NO-SAVE-RESTORE
 
 // SAVE-RESTORE: warning: the clang compiler does not support '-msave-restore'
-// DEFAULT-NOT: warning: the clang compiler does not support
-
-// SAVE-RESTORE: "-target-feature" "+save-restore"
-// NO-SAVE-RESTORE: "-target-feature" "-save-restore"
-// DEFAULT: "-target-feature" "-save-restore"
-// DEFAULT-NOT: "-target-feature" "+save-restore"
+// NO-SAVE-RESTORE-NOT: warning: the clang compiler does not support
+// DEFAULT-NOT: warning: the clang compiler does not support