[Sema] Add check for bitfield assignments to integral types (#69049)

This change introduces the bitfield conversion check after fixes to the
test case. The original submission unfortunately did not comprehend
differences in architecture for the diagnostic messages, leading to
unanticipated failures in the arm build bots.

Original PR: https://github.com/llvm/llvm-project/pull/68276

Clang does not check for bitfield assignment widths, while gcc checks
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.