chore: only ignore c++ diagnostics for ddb headers

[0ax1]

DDB warnings to ignore need to be part of the DUCKDB_INCLUDES_BEGIN macro:

#define DUCKDB_INCLUDES_BEGIN                                                  \
    _Pragma("GCC diagnostic push")                                             \
    _Pragma("GCC diagnostic ignored \"-Wall\"")                                \
    _Pragma("GCC diagnostic ignored \"-Wextra\"")                              \
    _Pragma("GCC diagnostic ignored \"-Wpedantic\"")                           \
    _Pragma("GCC diagnostic ignored \"-Wunused-parameter\"")                   \
    _Pragma("GCC diagnostic ignored \"-Wtype-limits\"")
#define DUCKDB_INCLUDES_END _Pragma("GCC diagnostic pop")

Warnings originating from our own code should simply be addressed.

[codspeed-hq]

Merging this PR will improve performance by 18.36%

⚡ 2 improved benchmarks
✅ 1007 untouched benchmarks
⏩ 1515 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	376.1 ns	317.8 ns	+18.36%
⚡	Simulation	bitwise_not_vortex_buffer_mut[1024]	535.6 ns	477.2 ns	+12.22%

---

_{Comparing ad/only-ignore-diagnostics-for-ddb-headers (5f1fb8b) with develop (d08c89b)}

Footnotes

1. 1515 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩