Fix Windows LTO: preserve required procedures with llvm.used

On Windows with LTO, required procedures with external linkage need to
be added to @llvm.used to survive linker-level dead code elimination.

LLVM may generate implicit calls to runtime builtins (e.g., __extendhfsf2
for f16 conversions) during instruction lowering, after the IR is
finalized. Without @llvm.used, the linker discards these procedures
before the implicit calls are generated.

This adds required procedures to @llvm.used at creation time. The fix
is Windows-specific; other platforms handle this correctly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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