DX-105463: [C++][Gandiva] Add TimestampIR for unit-aware timestamp[us/ns] support

[dmitry-chirkov-dremio]

Summary

Adds TimestampIR — an LLVM IR builder class modeled after DecimalIR — that generates unit-aware wrapper functions at module-load time, enabling Gandiva functions registered for timestamp[ms] to automatically handle timestamp[us] and timestamp[ns] inputs without explicit per-unit registry entries.

What changes are included in this PR?

• timestamp_ir.h/cc (new): TimestampIR class with wrapper patterns: pure IR arithmetic, calendar split/recombine, extract, trunc, diff, cast, and timezone wrappers. Includes FloorDiv/FloorDivRem helpers for correct floor-toward-negative-infinity semantics on pre-epoch timestamps.
• CMakeLists.txt + engine.cc: Wire TimestampIR::AddFunctions() at module load alongside DecimalIR.
• function_signature.cc: DataTypeEquals for TIMESTAMP ignores TimeUnit; only timezone is significant for matching. This allows the existing timestamp[ms] registry entries to match calls with timestamp[us]/timestamp[ns] parameters.
• llvm_generator.cc/h: BuildFunctionCall inspects function descriptor for non-ms timestamp params and remaps to _us/_ns suffixed IR functions. Propagates Status errors from the visitor back to the caller (previously silently dropped).
• precompiled/time.cc: Floor-division fix in DATE_TRUNC_FIXED_UNIT for pre-epoch (negative) timestamps; fix sub-second millis sign in castVARCHAR_timestamp_int64 for negative timestamps.
• tests/date_time_test.cc: End-to-end C++ tests for timestamp[us] and timestamp[ns] through extract, trunc, arithmetic, and cast functions.

Are these changes tested?

Yes — tests/date_time_test.cc covers the new unit-aware paths. Also validated end-to-end through arrow-java ProjectorTest and Dremio integration tests (separate PRs).

Are there any user-facing changes?

No API changes. Gandiva functions that previously only accepted timestamp[ms] now transparently accept timestamp[us] and timestamp[ns].

Safe degradation: If a caller uses an old native lib (pre-TimestampIR), DataTypeEquals falls back to strict matching, function lookup fails at validation time, and the caller gets a clean error rather than silent wrong results.