[Bounds Safety] Add -fexperimental-bounds-safety CC1 and language option and use it to tweak counted_by's semantics

This adds the `-fexperimental-bounds-safety` CC1 and corresponding
language option. This language option enables "-fbounds-safety" which
is a bounds-safety extension for C that is being incrementally
upstreamed.

This CC1 flag is not exposed as a driver flag yet because most of the
implementation isn't upstream yet.

The language option is used to make a small semantic change to how the
`counted_by` attribute is treated. Without
`-fexperimental-bounds-safety` the attribute is allowed (but emits a
warning) on a flexible array member where the element type is a struct
with a flexible array member. With the flag this situation is an error.

E.g.

```
struct has_unannotated_FAM {
  int count;
  char buffer[];
};

struct buffer_of_structs_with_unnannotated_FAM {
  int count;
  // Forbidden with `-fexperimental-bounds-safety`
  struct has_unannotated_FAM Arr[] __counted_by(count);
};
```

The above code **should always** be an error. However, when #90786 was
originally landed (which allowed `counted_by` to be used on pointers in
structs) it exposed an issue in code in the Linux kernel that was using
the `counted_by` attribute incorrectly (see
#90786 (comment))
which was now caught by a new error diagnostic in the PR. To unbreak the
build of the Linux kernel the error diagnostic was temporarily
downgraded to be a warning to give the kernel authors time to fix their
code.

This downgrading of the error diagnostic to a warning is a departure
from the intended semantics of `-fbounds-safety` so in order to have
both behaviors (error and warning) it is necessary for Clang to actually
have a notion of `-fbounds-safety` being on vs off.

rdar://125400392

Author: delcypher

clang/include/clang/Basic/LangOptions.def

@@ -520,6 +520,9 @@ BENIGN_LANGOPT(CheckNew, 1, 0, "Do not assume C++ operator new may not return NU
 BENIGN_LANGOPT(CheckConstexprFunctionBodies, 1, 1,
                "Emit diagnostics for a constexpr function body that can never "
                "be used in a constant expression.")
+
+LANGOPT(BoundsSafety, 1, 0, "Bounds safety extension for C")
+
 #undef LANGOPT
 #undef COMPATIBLE_LANGOPT
 #undef BENIGN_LANGOPT

clang/include/clang/Driver/Options.td

@@ -1911,6 +1911,14 @@ def fapinotes_swift_version : Joined<["-"], "fapinotes-swift-version=">,
   MetaVarName<"<version>">,
   HelpText<"Specify the Swift version to use when filtering API notes">;
 
+defm bounds_safety : BoolFOption<
+  "experimental-bounds-safety",
+  LangOpts<"BoundsSafety">, DefaultFalse,
+  PosFlag<SetTrue, [], [CC1Option], "Enable">,
+  NegFlag<SetFalse, [], [CC1Option], "Disable">,
+  BothFlags<[], [CC1Option],
+          " experimental bounds safety extension for C">>;
+
 defm addrsig : BoolFOption<"addrsig",
   CodeGenOpts<"Addrsig">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option], "Emit">,

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5948,7 +5948,7 @@ CheckCountedByAttrOnField(Sema &S, FieldDecl *FD, Expr *E,
   } else if (PointeeTy->isFunctionType()) {
     InvalidTypeKind = CountedByInvalidPointeeTypeKind::FUNCTION;
   } else if (PointeeTy->isStructureTypeWithFlexibleArrayMember()) {
-    if (FieldTy->isArrayType()) {
+    if (FieldTy->isArrayType() && !S.getLangOpts().BoundsSafety) {
       // This is a workaround for the Linux kernel that has already adopted
       // `counted_by` on a FAM where the pointee is a struct with a FAM. This
       // should be an error because computing the bounds of the array cannot be

clang/test/Sema/attr-counted-by-bounds-safety-vlas.c

@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -fsyntax-only -fexperimental-bounds-safety -verify %s
+//
+// This is a portion of the `attr-counted-by-vla.c` test but is checked
+// under the semantics of `-fexperimental-bounds-safety` which has different
+// behavior.
+
+#define __counted_by(f)  __attribute__((counted_by(f)))
+
+struct has_unannotated_VLA {
+  int count;
+  char buffer[];
+};
+
+struct has_annotated_VLA {
+  int count;
+  char buffer[] __counted_by(count);
+};
+
+struct buffer_of_structs_with_unnannotated_vla {
+  int count;
+  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'struct has_unannotated_VLA' is a struct type with a flexible array member}}
+  struct has_unannotated_VLA Arr[] __counted_by(count);
+};
+
+
+struct buffer_of_structs_with_annotated_vla {
+  int count;
+  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'struct has_annotated_VLA' is a struct type with a flexible array member}}
+  struct has_annotated_VLA Arr[] __counted_by(count);
+};
+
+struct buffer_of_const_structs_with_annotated_vla {
+  int count;
+  // Make sure the `const` qualifier is printed when printing the element type.
+  // expected-error@+1{{'counted_by' cannot be applied to an array with element of unknown size because 'const struct has_annotated_VLA' is a struct type with a flexible array member}}
+  const struct has_annotated_VLA Arr[] __counted_by(count);
+};