Allow linking the system SQLite on Linux

[colemancda]

Allow Linux to link the system SQLite instead of the embedded swift-sqlcipher build.

Changes

• Add a CSQLite system library target (Sources/CSQLite) that wraps <sqlite3.h> and links sqlite3, declared with pkgConfig: "sqlite3" and apt/yum providers.
• SWIFT_BUILD_SYSTEM_SQLITE=1 opts Linux into it. The target is only declared in that case, so a default build never performs a pkg-config lookup.
• The Linux branch of the import chain changed from SwiftToolchainCSQLite to CSQLite. The old module is toolchain-internal (swift-toolchain-sqlite) and not a supported import for package code; this makes the dependency explicit and declared.
• Add a Linux (system SQLite) CI job that installs libsqlite3-dev and pkg-config and builds with the opt-in set. pkg-config is required by the pkgConfig: declaration, not just the headers.
• README documents the opt-in.

Defaults

Unchanged. Linux still uses the embedded build, as do Android, Windows, WASI, and OpenBSD; Darwin still links SQLite3.framework. Nothing here is a source break — consumers who don't set the env var see no difference, and the existing Linux CI job still covers that path.

Testing

• macOS: builds clean, all 124 tests pass.
• Manifest evaluates in both modes, with CSQLite present in the target list only under SWIFT_BUILD_SYSTEM_SQLITE=1.
• The Linux side of both modes is what the two Linux CI jobs on this PR will confirm.

[github-code-quality]

Code Coverage Overview

Languages: Swift

Swift / code-coverage/llvm-cov

The overall coverage in commit 2752e9e in the feature/linux-system... branch remains at 96%, unchanged from commit 19b9b5a in the master branch.

---

_{Updated July 28, 2026 18:28 UTC}