<exception>: Optimize exception_ptr operations - #6403

Summary

Optimizes several std::exception_ptr operations that currently call out-of-line CRT helpers even though the representation is already known in <exception>.

This change:

• makes the default constructor and nullptr_t constructor rely on the existing null member initializers
• implements operator bool() as _Data1 != nullptr
• compares _Data1 directly in operator==
• swaps _Data1 and _Data2 directly
• implements assignment from nullptr_t using swap-with-empty, avoiding __ExceptionPtrAssign

The exported CRT helper functions remain unchanged for ABI compatibility.

Fixes #6387.

Rationale

exception_ptr is intentionally laid out to match shared_ptr<const _EXCEPTION_RECORD>. Its two data members correspond to the shared pointer's stored pointer and control block, and the implementation already contains a static_assert enforcing matching size and alignment.

The removed out-of-line calls currently perform operations equivalent to the inlined implementations above:

• __ExceptionPtrCreate default-constructs an empty shared_ptr
• __ExceptionPtrToBool converts the underlying shared_ptr to bool
• __ExceptionPtrCompare compares the underlying shared_ptr
• __ExceptionPtrSwap swaps the underlying shared_ptr

For operator=(nullptr_t), swapping with an empty exception_ptr preserves ownership semantics and allows the temporary's destructor to release the previous control block correctly.

Validation

The existing Dev11_0299014_exception_ptr_requirements test already exercises the affected semantics, including:

• default and nullptr construction
• bool conversion
• equality between empty, aliased, and distinct exception pointers
• assignment from nullptr
• swapping
• self-swap

No ABI-visible layout or exported CRT symbol is changed.