[Clang] Separate implicit int conversion on negation sign to new diagnostic group (#139429)

This PR reverts a change made in #126846. 

#126846 introduced an ``-Wimplicit-int-conversion`` diagnosis for 
```c++
int8_t x = something;
x = -x; // warning here
```

This is technically correct since -x could have a width of 9, but this
pattern is common in codebases.

Reverting this change would also introduce the false positive I fixed in
#126846:
```c++
bool b(signed char c) {
  return -c >= 128; // -c can be 128
}
```

This false positive is uncommon, so I think it makes sense to revert the
change.

0 files changed, 0 insertions, 0 deletions