[Assignment Tracking] Fix -fexperimental-assignment-tracking cc1 flag

[Orlando]

Summary

This change allows -fexperimental-assignment-tracking to be negated using the negative equivalent -fno-experimental-assignment-tracking, and fixes the flag so that the default behaviour can be changed (currently the default is still false / disabled).

clang/include/clang/Driver/Options.td

 def fdenormal_fp_math_f32_EQ : Joined<["-"], "fdenormal-fp-math-f32=">,
    Group<f_Group>;
 
 def fctor_dtor_return_this : Flag<["-"], "fctor-dtor-return-this">,
   HelpText<"Change the C++ ABI to returning `this` pointer from constructors "
            "and non-deleting destructors. (No effect on Microsoft ABI)">,
   MarshallingInfoFlag<CodeGenOpts<"CtorDtorReturnThis">>;
 
-def fexperimental_assignment_tracking :
-  Flag<["-"], "fexperimental-assignment-tracking">, Group<f_Group>,
-  HelpText<"Enable assignment tracking debug info">,
-  MarshallingInfoFlag<CodeGenOpts<"EnableAssignmentTracking">>;
+defm experimental_assignment_tracking :
+  BoolOption<"f", "experimental-assignment-tracking",
+  CodeGenOpts<"EnableAssignmentTracking">, DefaultFalse,
+  PosFlag<SetTrue, [CC1Option]>, NegFlag<SetFalse>, BothFlags<[CoreOption]>>,
+  Group<f_Group>;
 
 } // let Flags = [CC1Option, NoDriverOption]
 
 //===----------------------------------------------------------------------===//
 // Dependency Output Options
 //===----------------------------------------------------------------------===//
 
 let Flags = [CC1Option, NoDriverOption] in {

clang/test/CodeGen/assignment-tracking/flag.cpp

+//// Explicitly enabled:
 // RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
 // RUN:     -emit-llvm  -fexperimental-assignment-tracking %s -o -               \
-// RUN: | FileCheck %s --check-prefixes=FLAG
+// RUN: | FileCheck %s --check-prefixes=ENABLE
+//// Disabled by default:
 // RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
 // RUN:     -emit-llvm  %s -o -                                                  \
-// RUN: | FileCheck %s --check-prefixes=NO-FLAG
+// RUN: | FileCheck %s --check-prefixes=DISABLE
+//// Explicitly disabled:
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
+// RUN:     -emit-llvm  %s -o - -fno-experimental-assignment-tracking            \
+// RUN: | FileCheck %s --check-prefixes=DISABLE
 
 // Check some assignment-tracking stuff appears in the output when the flag
-// -fexperimental-assignment-tracking is used, and that it doesn't when
-// the flag is not used (default behaviour: no assignment tracking).
+// -fexperimental-assignment-tracking is used, that it doesn't when
+// -fno-experimental-assignment-tracking is used or neither flag is specified.
 
-// FLAG: DIAssignID
-// FLAG: dbg.assign
+// ENABLE: DIAssignID
+// ENABLE: dbg.assign
 
-// NO-FLAG-NOT: DIAssignID
-// NO-FLAG-NOT: dbg.assign
+// DISABLE-NOT: DIAssignID
+// DISABLE-NOT: dbg.assign
 
 void fun(int a) {}