[Sema] Add check for bitfield assignments to larger integral types (#68276)

We noticed that clang does not check for bitfield assignment widths,
while gcc does check this.

gcc produced a warning like so for it's -Wconversion flag:
```
$ gcc -Wconversion -c test.c
test.c: In function 'foo':
test.c:10:15: warning: conversion from 'int' to 'signed char:7' may change value [-Wconversion]
   10 |      vxx.bf = x; // no warning
      |               ^
```

This change simply adds this check for integral types under the
-Wbitfield-conversion compiler option.