[CharacterSet] Replace withUnsafePointer with C-helper method #1262

Job	Run time
Build (CMake)	15m 13s
Test (SwiftPM) / Windows (nightly-main - windows-2022)	23m 48s
Test (SwiftPM) / Swift SDK for Android Build (nightly-main - NDK r28c)	15m 39s
Test (SwiftPM) / Swift SDK for Android Build (nightly-main - NDK r27d)	13m 32s
Test (SwiftPM) / Swift SDK for Wasm Build (nightly-main - jammy)	7m 51s
Test (SwiftPM) / Linux (nightly-main - jammy - x86_64)	9m 4s
Soundness / Broken symlinks check	5s
Soundness / Python lint check	7s
Soundness / Shell check	28s
Soundness / YAML lint check	10s
Test (SwiftPM) / Embedded Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test (SwiftPM) / FreeBSD (${{ matrix.swift_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Soundness / Format check	0s
Test (SwiftPM) / macOS (Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Test (SwiftPM) / Static Linux Swift SDK Build (${{ matrix.swift_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Soundness / Unacceptable language check	0s
Soundness / Documentation check	0s
Soundness / Documentation check (macOS)	0s
Test (SwiftPM) / iOS (Build Only, Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Soundness / API breakage check	0s
Soundness / License headers check	0s
	1h 25m 57s

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