[Velox] Fix Iceberg writer aggregate-init build failure on macOS C++20 libc++ (#12115)

Two call sites in cpp/velox/compute/iceberg/IcebergWriter.cc construct
aggregate types via paren-init with multiple arguments:

  fields.emplace_back(name, type, transform, parameter);   // line 311
  return WriteStats(numWrittenBytes,                       // line 262
                    numWrittenFiles, writeIOTimeNs,
                    writeWallNs);

Both target types are pure aggregate structs without a user-defined
constructor:

  // Velox: velox/connectors/hive/iceberg/PartitionSpec.h
  struct IcebergPartitionSpec::Field {
    std::string name;
    TypePtr type;
    TransformType transformType;
    std::optional<int32_t> parameter;
  };

  // Gluten: cpp/velox/compute/iceberg/IcebergWriter.h
  struct WriteStats {
    uint64_t numWrittenBytes{0};
    uint32_t numWrittenFiles{0};
    uint64_t writeIOTimeNs{0};
    uint64_t writeWallNs{0};
    ...
  };

C++20 P0960R3 made aggregates initializable via paren-init at the
language level, but the standard library implementation of
std::construct_at (which std::vector::emplace_back routes through to do
in-place construction) is uneven across stdlibs:

  - libstdc++ (Linux GCC): construct_at accepts aggregate paren-init,
    so emplace_back(a, b, c, d) compiles for aggregate types.
  - libc++ (Apple Clang 17, macOS): construct_at still requires a real
    constructor, so the substitution fails:

      error: no matching function for call to 'construct_at'
      note: candidate template ignored: substitution failure
      [with _Tp = IcebergPartitionSpec::Field, _Args = ...]:
      no matching constructor for initialization of
      'IcebergPartitionSpec::Field'

Result: builds with --enable_enhanced_features=ON on macOS abort
hard at IcebergWriter.cc.cpp.o (~156/217) before producing
libgluten.dylib, blocking any macOS user from running the Iceberg
writer integration that this gate adds.

The fix is a minimal style change at the two call sites — switch
from paren-init to brace-init, which is aggregate initialization on
every C++17/20 toolchain (libstdc++, libc++, MSVC):

  fields.push_back({name, type, transform, parameter});
  return WriteStats{numWrittenBytes, numWrittenFiles, ...};

Linux libstdc++ continues to compile cleanly: brace-init resolves to
the same aggregate initialization the paren-init form selected via
P0960. Zero runtime change, identical field-by-field initialization.

Verification:
- macOS 14 arm64 + Apple Clang 17 + --enable_enhanced_features=ON:
  cpp build now reaches 217/217 (was 156/217 before this fix).
- cpp ctest matrix on the same configuration: 5539 / 5539 pass,
  including the enhanced-only velox_iceberg_test executable.
- Linux x86_64 build remains green; brace-init for aggregates is
  unchanged in libstdc++.

Co-authored-by: Kent Yao <kentyao@microsoft.com>

Author: jackylee-ch

cpp/velox/compute/iceberg/IcebergWriter.cc

@@ -259,11 +259,11 @@ WriteStats IcebergWriter::writeStats() const {
   const auto currentTimeNs = getCurrentTimeNano();
   VELOX_CHECK_GE(currentTimeNs, createTimeNs_);
   const auto sinkStats = dataSink_->stats();
-  return WriteStats(
+  return WriteStats{
       sinkStats.numWrittenBytes,
       sinkStats.numWrittenFiles,
       sinkStats.writeIOTimeUs * 1000,
-      currentTimeNs - createTimeNs_);
+      currentTimeNs - createTimeNs_};
 }
 
 std::shared_ptr<const iceberg::IcebergPartitionSpec>
@@ -308,7 +308,7 @@ parseIcebergPartitionSpec(const uint8_t* data, const int32_t length, RowTypePtr
       parameter = protoField.parameter();
     }
 
-    fields.emplace_back(protoField.name(), rowType->findChild(protoField.name()), transform, parameter);
+    fields.push_back({protoField.name(), rowType->findChild(protoField.name()), transform, parameter});
   }
 
   return std::make_shared<iceberg::IcebergPartitionSpec>(protoSpec.spec_id(), fields);