Fix "[clang][UBSan] Add implicit conversion check for bitfields" (#87761)

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>