Fix since #75481 got reverted.

- Explicitly set BitfieldBits to 0 to avoid uninitialized field member
for the integer checks:
```diff
-       llvm::ConstantInt::get(Builder.getInt8Ty(), Check.first)};
+      llvm::ConstantInt::get(Builder.getInt8Ty(), Check.first),
+      llvm::ConstantInt::get(Builder.getInt32Ty(), 0)};
```
- `Value **Previous` was erroneously `Value *Previous` in
`CodeGenFunction::EmitWithOriginalRHSBitfieldAssignment`, fixed now.
- Update following:
```diff
-     if (Kind == CK_IntegralCast) {
+     if (Kind == CK_IntegralCast || Kind == CK_LValueToRValue) {
```
CK_LValueToRValue when going from, e.g., char to char, and
CK_IntegralCast otherwise.
- Make sure that `Value *Previous = nullptr;` is initialized (see
https://github.com/llvm/llvm-project/commit/1189e87951e59a81ee097eae847c06008276fef1)
- Add another extensive testcase
`ubsan/TestCases/ImplicitConversion/bitfield-conversion.c`

---------

Co-authored-by: Vitaly Buka <vitalybuka@gmail.com>

Conversion performed using the script at:
https://gist.github.com/nikic/98357b71fd67756b0f064c9517b62a34

These are only tests where no manual fixup was required.

This adds -no-opaque-pointers to clang tests whose output will
change when opaque pointers are enabled by default. This is
intended to be part of the migration approach described in
https://discourse.llvm.org/t/enabling-opaque-pointers-by-default/61322/9.

The patch has been produced by replacing %clang_cc1 with
%clang_cc1 -no-opaque-pointers for tests that fail with opaque
pointers enabled. Worth noting that this doesn't cover all tests,
there's a remaining ~40 tests not using %clang_cc1 that will need
a followup change.

Differential Revision: https://reviews.llvm.org/D123115

Sometimes people get minimal crash reports after a UBSAN incident. This change
tags each trap with an integer representing the kind of failure encountered,
which can aid in tracking down the root cause of the problem.

Summary:
As reported by @regehr (thanks!) on twitter (https://twitter.com/johnregehr/status/1057681496255815686),
we (me) has completely forgot about the binary assignment operator.
In AST, it isn't represented as separate `ImplicitCastExpr`'s,
but as a single `CompoundAssignOperator`, that does all the casts internally.
Which means, out of these two, only the first one is diagnosed:
```
auto foo() {
    unsigned char c = 255;
    c = c + 1;
    return c;
}
auto bar() {
    unsigned char c = 255;
    c += 1;
    return c;
}
```
https://godbolt.org/z/JNyVc4

This patch does handle the `CompoundAssignOperator`:
```
int main() {
  unsigned char c = 255;
  c += 1;
  return c;
}
```
```
$ ./bin/clang -g -fsanitize=integer /tmp/test.c && ./a.out
/tmp/test.c:3:5: runtime error: implicit conversion from type 'int' of value 256 (32-bit, signed) to type 'unsigned char' changed the value to 0 (8-bit, unsigned)
    #0 0x2392b8 in main /tmp/test.c:3:5
    #1 0x7fec4a612b16 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x22b16)
    #2 0x214029 in _start (/build/llvm-build-GCC-release/a.out+0x214029)
```

However, the pre/post increment/decrement is still not handled.

Reviewers: rsmith, regehr, vsk, rjmccall, #sanitizers

Reviewed By: rjmccall

Subscribers: mclow.lists, cfe-commits, regehr

Tags: #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D53949

llvm-svn: 347258