[Clang] Add warnings when mixing different charN_t types (#138708)

charN_t represent code units of different UTF encodings. Therefore the
values of 2 different charN_t objects do not represent the same
characters.

In order to avoid comparing apples and oranges, we add new warnings to
warn on:
  - Implicit conversions
  - Comparisons
  - Other cases involving arithmetic conversions

We only produce the warning if we cannot establish the comparison would
be safe through constant evaluation.

The new `-Wimplicit-unicode-conversion` warning is enabled by default.

Note that this PR intentionally doesn;t touches char/wchar_t, but it
would be worth considering also warning on extending the new warnings to
these types (in a follow up)

Additionally most arithmetic operations on charN_t don't really make
sense (ie what does it mean to addition code units), so we could add
warnings for that.

Fixes #138526

0 files changed, 0 insertions, 0 deletions