Issue Description:

Sourcekitd's AST reuse for code completion appears to be fragile when relative paths and/or -working-dir arguments are being used.

When this issues appears, code completion shows incrementality/AST reuse (visible by setting SOURCEKIT_LOGGING=3 and checking for the reusingastcontext flag in the sourcekit code completion reply) for ~5 seconds or so, and then loses incrementality, making something in this method the likely cause (https://github.com/apple/swift/blob/c9dbef61b2122277e42d450f34ec0f3be4fcea95/lib/IDE/CompletionInstance.cpp#L235).

Since Xcode uses absolute paths for its arguments, this is only easily reproducible in sourcekit-lsp.

Steps to reproduce:

1. Build sourcekit-lsp from my fork which changes sourcekit-lsp's build arguments to use relative path handling; you could probably do the same for Xcode's SPM integration.

2. Checkout and build https://github.com/apple/swift-llbuild2 using swift build

3. Open the swift-llbuild2 checkout using an editor hooked up to your custom sourcekit-lsp (and make sure it's using the same toolchain you used to build the swift-llbuild2) and make sure you've set SOURCEKIT_LOGGING=3 to enable sourcekit logging.

4. Navigate to https://github.com/apple/swift-llbuild2/blob/85b95bcd53b499af567d9b85cb5cdfe72b78fbbc/Sources/llbuild2/Core/Engine.swift#L55 and trigger code completion by copying and pasting "self" and then typing .

5. Observe from the sourcekit logs that the first code completion isn't reusing the AST context. If you do a subsequent code completion within 5 seconds, it will reuse the AST context, if you wait longer than 5 seconds, the next code completion will not reuse the AST context.