[Driver] Add -gno-split-dwarf which can disable debug fission

[MaskRay]

Summary

Currently when -gsplit-dwarf is specified (could be buried in a build system),
there is no convenient way to cancel debug fission without affecting the debug
information amount (all of -g0, -g1 -fsplit-dwarf-inlining, and -gline-directives-only
can, but they affect the debug information amount).

[dblaikie]

Looks good, thanks!

[MaskRay]

Test there is no -ggnu-pubnames

clang/include/clang/Driver/Options.td

 def gstrict_dwarf : Flag<["-"], "gstrict-dwarf">, Group<g_flags_Group>;
 def gno_strict_dwarf : Flag<["-"], "gno-strict-dwarf">, Group<g_flags_Group>;
 def gcolumn_info : Flag<["-"], "gcolumn-info">, Group<g_flags_Group>, Flags<[CoreOption]>;
 def gno_column_info : Flag<["-"], "gno-column-info">, Group<g_flags_Group>, Flags<[CoreOption, CC1Option]>;
 def gsplit_dwarf : Flag<["-"], "gsplit-dwarf">, Group<g_flags_Group>;
 def gsplit_dwarf_EQ : Joined<["-"], "gsplit-dwarf=">, Group<g_flags_Group>,
   HelpText<"Set DWARF fission mode to either 'split' or 'single'">,
   Values<"split,single">;
+def gno_split_dwarf : Flag<["-"], "gno-split-dwarf">, Group<g_flags_Group>;
 def ggnu_pubnames : Flag<["-"], "ggnu-pubnames">, Group<g_flags_Group>, Flags<[CC1Option]>;
 def gno_gnu_pubnames : Flag<["-"], "gno-gnu-pubnames">, Group<g_flags_Group>;
 def gpubnames : Flag<["-"], "gpubnames">, Group<g_flags_Group>, Flags<[CC1Option]>;
 def gno_pubnames : Flag<["-"], "gno-pubnames">, Group<g_flags_Group>;
 def gdwarf_aranges : Flag<["-"], "gdwarf-aranges">, Group<g_flags_Group>;
 def gmodules : Flag <["-"], "gmodules">, Group<gN_Group>,
   HelpText<"Generate debug info with external references to clang modules"
            " or precompiled headers">;

clang/lib/Driver/ToolChains/Clang.cpp

if (!Args.hasFlag(options::OPT_fspell_checking,
                     options::OPT_fno_spell_checking))
     CmdArgs.push_back("-fno-spell-checking");
 }
 
 enum class DwarfFissionKind { None, Split, Single };
 
 static DwarfFissionKind getDebugFissionKind(const Driver &D,
                                             const ArgList &Args, Arg *&Arg) {
-  Arg =
-      Args.getLastArg(options::OPT_gsplit_dwarf, options::OPT_gsplit_dwarf_EQ);
-  if (!Arg)
+  Arg = Args.getLastArg(options::OPT_gsplit_dwarf, options::OPT_gsplit_dwarf_EQ,
+                        options::OPT_gno_split_dwarf);
+  if (!Arg || Arg->getOption().matches(options::OPT_gno_split_dwarf))
     return DwarfFissionKind::None;
 
   if (Arg->getOption().matches(options::OPT_gsplit_dwarf))
     return DwarfFissionKind::Split;
 
   StringRef Value = Arg->getValue();
   if (Value == "split")
     return DwarfFissionKind::Split;

clang/test/Driver/split-debug.c

 // SPLIT:      "-debug-info-kind=limited"
 // SPLIT-SAME: "-ggnu-pubnames"
 // SPLIT-SAME: "-split-dwarf-file" "split-debug.dwo" "-split-dwarf-output" "split-debug.dwo"
 
 // RUN: %clang -### -c -target wasm32 -gsplit-dwarf -g %s 2>&1 | FileCheck %s --check-prefix=SPLIT
 // RUN: %clang -### -c -target amdgcn-amd-amdhsa -gsplit-dwarf -g %s 2>&1 | FileCheck %s --check-prefix=SPLIT
 
 /// -gsplit-dwarf is a no-op on a non-ELF platform.
-// RUN: %clang -### -c -target x86_64-apple-darwin  -gsplit-dwarf -g %s 2>&1 | FileCheck %s --check-prefix=NOSPLIT
-// NOSPLIT-NOT: "-split-dwarf
+// RUN: %clang -### -c -target x86_64-apple-darwin  -gsplit-dwarf -g %s 2>&1 | FileCheck %s --check-prefix=DARWIN
+// DARWIN:     "-debug-info-kind=standalone"
+// DARWIN-NOT: "-split-dwarf
 
 /// -gsplit-dwarf is a no-op if no -g is specified.
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf %s 2>&1 | FileCheck %s --check-prefix=G0
 
+/// -gno-split-dwarf disables debug fission.
+// RUN: %clang -### -c -target x86_64 -gsplit-dwarf -g -gno-split-dwarf %s 2>&1 | FileCheck %s --check-prefix=NOSPLIT
+// RUN: %clang -### -c -target x86_64 -gsplit-dwarf=single -g -gno-split-dwarf %s 2>&1 | FileCheck %s --check-prefix=NOSPLIT
+// RUN: %clang -### -c -target x86_64 -gno-split-dwarf -g -gsplit-dwarf %s 2>&1 | FileCheck %s --check-prefixes=NOINLINE,SPLIT
+
+// NOSPLIT:     "-debug-info-kind=limited"
+// NOSPLIT-NOT: "-ggnu-pubnames"
+// NOSPLIT-NOT: "-split-dwarf
+
 /// Test -gsplit-dwarf=single.
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf=single -g %s 2>&1 | FileCheck %s --check-prefix=SINGLE
 
 // SINGLE: "-debug-info-kind=limited"
 // SINGLE: "-split-dwarf-file" "split-debug.o"
 // SINGLE-NOT: "-split-dwarf-output"
 
 // RUN: %clang -### -c -target x86_64 -gsplit-dwarf=single -g -o %tfoo.o %s 2>&1 | FileCheck %s --check-prefix=SINGLE_WITH_FILENAME