[SystemZ] Enable `-mtune=` option in clang.

[Kai]

Summary

D128910 enabled handling of attribute "tune-cpu" .
This PR now enables option -mtune option, which then generates the
new attribute in clang.

[uweigand]

LGTM, thanks!

clang/docs/ClangCommandLineReference.rst

 .. option:: -mtls-size=<arg>
 
 Specify bit size of immediate TLS offsets (AArch64 ELF only): 12 (for 4KB) \| 24 (for 16MB, default) \| 32 (for 4GB) \| 48 (for 256TB, needs -mcmodel=large)
 
 .. program:: clang1
 .. option:: -mtune=<arg>
 .. program:: clang
 
-Only supported on X86 and RISC-V. Otherwise accepted for compatibility with GCC.
+Only supported on X86, RISC-V and SystemZ. Otherwise accepted for compatibility with GCC.
 
 .. option:: -mtvos-version-min=<arg>, -mappletvos-version-min=<arg>
 
 .. option:: -municode<arg>
 
 .. option:: -munsafe-fp-atomics, -mno-unsafe-fp-atomics
 
 Enable unsafe floating point atomic instructions (AMDGPU only)

clang/include/clang/Driver/Options.td

def mno_relax_pic_calls : Flag<["-"], "mno-relax-pic-calls">,
   HelpText<"Do not produce relaxation hints for linkers to try optimizing PIC "
            "call sequences into direct calls (MIPS only)">, Flags<[HelpHidden]>;
 def mglibc : Flag<["-"], "mglibc">, Group<m_libc_Group>, Flags<[HelpHidden]>;
 def muclibc : Flag<["-"], "muclibc">, Group<m_libc_Group>, Flags<[HelpHidden]>;
 def module_file_info : Flag<["-"], "module-file-info">, Flags<[NoXarchOption,CC1Option]>, Group<Action_Group>,
   HelpText<"Provide information about a particular module file">;
 def mthumb : Flag<["-"], "mthumb">, Group<m_Group>;
 def mtune_EQ : Joined<["-"], "mtune=">, Group<m_Group>,
-  HelpText<"Only supported on X86 and RISC-V. Otherwise accepted for compatibility with GCC.">;
+  HelpText<"Only supported on X86, RISC-V and SystemZ. Otherwise accepted for compatibility with GCC.">;
 def multi__module : Flag<["-"], "multi_module">;
 def multiply__defined__unused : Separate<["-"], "multiply_defined_unused">;
 def multiply__defined : Separate<["-"], "multiply_defined">;
 def mwarn_nonportable_cfstrings : Flag<["-"], "mwarn-nonportable-cfstrings">, Group<m_Group>;
 def canonical_prefixes : Flag<["-"], "canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
   HelpText<"Use absolute paths for invoking subcommands (default)">;
 def no_canonical_prefixes : Flag<["-"], "no-canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
   HelpText<"Use relative paths for invoking subcommands">;

clang/lib/Basic/Targets/SystemZ.h

public:
   int getISARevision(StringRef Name) const;
 
   bool isValidCPUName(StringRef Name) const override {
     return getISARevision(Name) != -1;
   }
 
   void fillValidCPUList(SmallVectorImpl<StringRef> &Values) const override;
 
+  bool isValidTuneCPUName(StringRef Name) const override {
+    return isValidCPUName(Name);
+  }
+
+  void fillValidTuneCPUList(SmallVectorImpl<StringRef> &Values) const override {
+    fillValidCPUList(Values);
+  }
+
   bool setCPU(const std::string &Name) override {
     CPU = Name;
     ISARevision = getISARevision(CPU);
     return ISARevision != -1;
   }
 
   bool
   initFeatureMap(llvm::StringMap<bool> &Features, DiagnosticsEngine &Diags,

clang/lib/Driver/ToolChains/Clang.cpp

if (FloatABI == sparc::FloatABI::Soft) {
     assert(FloatABI == sparc::FloatABI::Hard && "Invalid float abi!");
     CmdArgs.push_back("-mfloat-abi");
     CmdArgs.push_back("hard");
   }
 }
 
 void Clang::AddSystemZTargetArgs(const ArgList &Args,
                                  ArgStringList &CmdArgs) const {
-  bool HasBackchain = Args.hasFlag(options::OPT_mbackchain,
-                                   options::OPT_mno_backchain, false);
+  if (const Arg *A = Args.getLastArg(clang::driver::options::OPT_mtune_EQ)) {
+    StringRef Name = A->getValue();
+
+    std::string TuneCPU;
+    if (Name == "native")
+      TuneCPU = std::string(llvm::sys::getHostCPUName());
+    else
+      TuneCPU = std::string(Name);
+
+    if (!TuneCPU.empty()) {
+      CmdArgs.push_back("-tune-cpu");
+      CmdArgs.push_back(Args.MakeArgString(TuneCPU));
+    }
+  }
+
+  bool HasBackchain =
+      Args.hasFlag(options::OPT_mbackchain, options::OPT_mno_backchain, false);
   bool HasPackedStack = Args.hasFlag(options::OPT_mpacked_stack,
                                      options::OPT_mno_packed_stack, false);
   systemz::FloatABI FloatABI =
       systemz::getSystemZFloatABI(getToolChain().getDriver(), Args);
   bool HasSoftFloat = (FloatABI == systemz::FloatABI::Soft);
   if (HasBackchain && HasPackedStack && !HasSoftFloat) {
     const Driver &D = getToolChain().getDriver();
     D.Diag(diag::err_drv_unsupported_opt)