[ubsan] Make the 'vptr check disabled' warning more helpful

[vsk]

Summary

If -fsanitize=vptr is passed without -fsanitize=null being specified, it will say:

warning: implicitly disabling vptr sanitizer because null checking wasn't enabled (try specifying -fsanitize=null or -fsanitize=undefined)

Otherwise if the vptr check is enabled and null checking is explicitly disabled, it will say:

warning: implicitly disabling vptr sanitizer because null checking wasn't enabled (try removing -fno-sanitize=null)

See the mailing list thread re: r309007

[vsk]

Abandoned in favor of D36112, since it turns out -fsanitize=null,vptr doesn't work for some use cases.

include/clang/Basic/DiagnosticDriverKinds.td

def warn_debug_compression_unavailable : Warning<"cannot compress debug sections (zlib not installed)">,
   InGroup<DiagGroup<"debug-compression-unavailable">>;
 def warn_drv_enabling_rtti_with_exceptions : Warning<
   "implicitly enabling rtti for exception handling">,
   InGroup<DiagGroup<"rtti-for-exceptions">>;
 def warn_drv_disabling_vptr_no_rtti_default : Warning<
   "implicitly disabling vptr sanitizer because rtti wasn't enabled">,
   InGroup<AutoDisableVptrSanitizer>;
 def warn_drv_disabling_vptr_no_null_check : Warning<
-  "implicitly disabling vptr sanitizer because null checking wasn't enabled">,
+  "implicitly disabling vptr sanitizer because null checking wasn't enabled (try %select{specifying -fsanitize=null or -fsanitize=undefined|removing -fno-sanitize=null}0)">,
   InGroup<AutoDisableVptrSanitizer>;
 def warn_drv_object_size_disabled_O0 : Warning<
   "the object size sanitizer has no effect at -O0, but is explicitly enabled: %0">,
   InGroup<InvalidCommandLineArgument>;
 
 def note_drv_command_failed_diag_msg : Note<
   "diagnostic msg: %0">;
 def note_drv_t_option_is_global : Note<

lib/Driver/SanitizerArgs.cpp

if ((Kinds & Vptr) &&
        RTTIMode == ToolChain::RM_DisabledExplicitly)) {
     Kinds &= ~Vptr;
   }
 
   // Disable -fsanitize=vptr if -fsanitize=null is not enabled (the vptr
   // instrumentation is broken without run-time null checks).
   if ((Kinds & Vptr) && !(Kinds & Null)) {
     Kinds &= ~Vptr;
-    D.Diag(diag::warn_drv_disabling_vptr_no_null_check);
+    D.Diag(diag::warn_drv_disabling_vptr_no_null_check) << !!(AllRemove & Null);
   }
 
   // Check that LTO is enabled if we need it.
   if ((Kinds & NeedsLTO) && !D.isUsingLTO()) {
     D.Diag(diag::err_drv_argument_only_allowed_with)
         << lastArgumentForMask(D, Args, Kinds & NeedsLTO) << "-flto";
   }
 

test/Driver/fsanitize.c

 // CHECK-VPTR-TRAP-UNDEF: error: invalid argument '-fsanitize=vptr' not allowed with '-fsanitize-trap=undefined'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=vptr -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-RTTI
 // CHECK-VPTR-NO-RTTI: '-fsanitize=vptr' not allowed with '-fno-rtti'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-UNDEFINED-NO-RTTI
 // CHECK-UNDEFINED-NO-RTTI-NOT: vptr
 
-// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fno-sanitize=null %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-NULL
-// RUN: %clang -target x86_64-linux-gnu -fsanitize=vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-NO-NULL
-// CHECK-VPTR-NO-NULL: warning: implicitly disabling vptr sanitizer because null checking wasn't enabled
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=undefined -fno-sanitize=null %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-EXPLICIT-NO-NULL
+// CHECK-VPTR-EXPLICIT-NO-NULL: warning: implicitly disabling vptr sanitizer because null checking wasn't enabled (try removing -fno-sanitize=null)
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=vptr %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-VPTR-IMPLICIT-NO-NULL
+// CHECK-VPTR-IMPLICIT-NO-NULL: warning: implicitly disabling vptr sanitizer because null checking wasn't enabled (try specifying -fsanitize=null or -fsanitize=undefined)
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=address,thread -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANA-SANT
 // CHECK-SANA-SANT: '-fsanitize=address' not allowed with '-fsanitize=thread'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=address,memory -pie -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANA-SANM
 // CHECK-SANA-SANM: '-fsanitize=address' not allowed with '-fsanitize=memory'
 
 // RUN: %clang -target x86_64-linux-gnu -fsanitize=thread,memory -pie -fno-rtti %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-SANT-SANM