gh-148286: Fix undefined behaviour in the ctypes bit field accessors

[matthiasgoergens]

BIT_MASK, GET_BITFIELD and SET in Modules/_ctypes/cfield.c do their bit manipulation in the bit field's own type. When that type is signed, both shifting a negative value left and shifting a one into the sign bit are undefined behaviour, which is why Tools/ubsan/suppressions.txt carries two file-level entries for this file.

Removing those entries and running test_ctypes under UBSAN_OPTIONS=halt_on_error=1 aborts immediately at cfield.c:644 in i64_set. Collecting rather than halting shows diagnostics at three distinct lines, so the suppression was covering rather more than its comment documents:

cfield.c:644: left shift of 1 by 63 places cannot be represented in type 'int64_t'
cfield.c:644: left shift of negative value -9223372036854775793
cfield.c:640: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
cfield.c:640: left shift of 1 by 31 places cannot be represented in type 'int32_t'
cfield.c:636: left shift of negative value -32768

This does the arithmetic in uint64_t — the widest type the accessors support — and converts back to the field's own type at the end. BIT_MASK becomes LOW_BITS_MASK(), which no longer needs a type argument now that the mask is always computed unsigned; GET_BITFIELD gains the field type so it can still sign-extend signed fields, and is wrapped in do { } while (0).

Since this rewrites the arithmetic of every integer bit field getter and setter, I checked it exhaustively against the old macros rather than relying on review alone. For every fixed-width type from int8_t to uint64_t, every num_bits, every low_bit, ten storage bit patterns and twenty stored values — 1,167,600 cases — the old and new expressions produce identical results, under both gcc and clang. The same sweep under -fsanitize=undefined -fno-sanitize-recover=all is clean, where the old macros produce eleven distinct diagnostics.

With the two Modules/_ctypes/cfield.c entries removed from the suppressions file, test_ctypes passes under UBSAN_OPTIONS=halt_on_error=1.

• Issue: Various UB warnings found by UBSan #148286