[clang-tidy] Compact the confusable identifier table (#202626)

Encode confusable replacement strings once in a shared UTF-8 byte array.
Each sorted mapping becomes an eight-byte code point, 16-bit offset, and
16-bit length entry instead of reserving nineteen UTF-32 code points for
every replacement.

ConfusableIdentifierCheck appends the UTF-8 slice directly after the
binary search, removing the per-identifier UTF-32 to UTF-8 conversion.
Add compile-time assertions for the eight-byte entry layout and the 64
KiB replacement-data limit, plus generator-side overflow checks.

The logical table shrinks from 508,400 to 63,327 bytes, saving 445,073
bytes. A stripped standalone clang-tidy shrinks from 91,116,608 to
90,654,176 bytes, saving 462,432 bytes (0.508%).

A lookup microbenchmark improves from 79.9 to 40.4 ns median. A
100,000-identifier clang-tidy benchmark improves from 0.343 to 0.314
seconds mean.

Work towards #202616

AI tool disclosure: Co-authored with OpenAI Codex.